The older people thought they might be Japanese, remembering those who had fought against the British during the second world war for every inch of this monsoon-drenched land, but their language sounded harsher.
In fact, they were North Korean engineers building a complex of tunnels and bunkers for the Burmese military junta in an axis of outcasts.
More North Koreans were soon being seen — and even photographed by a daring local person — around Naypyidaw, the junta’s isolated new capital, where they oversaw labour gangs excavating a subterranean complex.
A third group of North Koreans has now been spotted in Chin state, bordering India and Bangladesh, an area full of restive minorities where few tourists venture. They are said to be equipping tunnels with generators and anti-gas ventilation systems.
Eventually the junta is to have a web of underground command posts, linked by fibreoptic cables, to help it put down any revolt and keep control in a national emergency, according to exiles from Burma and diplomats in Rangoon.
The North Koreans have sold similar services in the past to Saddam Hussein’s Iraq as well as to Iran and Syria.
One report suggested that some sections of tunnels in Burma were wide enough for trucks and could accommodate 600 personnel for several months, with storage space for food and weaponry.
Tunnelling is a military engineering skill that the North Koreans have perfected while honeycombing their fortress homeland with underground hangars and shelters against air or missile bombardment.
The North Koreans also specialise in air defence systems and radar. Their own installations were good enough to spot a Japanese electronic warfare aircraft taking off from its base in Honshu last month and to track its flight path all the way to the North Korean coast.
Such expertise has made them natural partners for the Burmese military, which shares their preoccupations with domestic security and foreign threats.
Yet it was only the presence of the North Korean workers that told Burmese onlookers of a secret relationship that has deepened as the two regimes have been pushed closer by international sanctions.
After exiles posted photographs on the internet, Burmese intelligence launched a "huge investigation" that led to the arrests of several journalists and the dismissal of senior officers, according to The Nation, a Thai newspaper. It quoted Thai intelligence officers, who watch Burma closely, confirming the existence of the tunnels.
The new alliance has required the leaders of Burma and North Korea to forget the bloody events of 1983, when a team of terrorists acting on the orders of Kim Jong-il blew up a ceremony welcoming the South Korean cabinet to Rangoon, killing 22 people, including ministers.
Since 2007, when the two re-established diplomatic relations after a 24-year break, shipping agents have noted a steady stream of vessels calling at the port of Thilawa.
The two pariah nations, all but cut off from global trade, appear to have agreed on a barter system. The North Korean ships have unloaded heavy equipment and wooden crates. Although dockside workers could not read the markings, they appeared to be military consignments, probably small arms and ammunition.
In return, the Burmese labourers piled rice, rubber, hardwood and rare minerals into the holds. Burma is rich in ores, including uranium.
Another advantage for North Korea is that goods can be transferred in Burmese ports to ships destined for Iran, frustrating attempts by the United States and its allies to watch and perhaps intercept military shipments.
Last week a mysterious North Korean ship, the Kang Nam 1, reversed course and steamed homewards after being trailed by an American Aegis-class destroyer, the USS John McCain, on a suspected voyage to Burma.
The CIA is principally concerned with perhaps the strangest of all Burma’s projects — its plan to operate a nuclear research reactor supplied by Rosatom, a Russian company, under an agreement announced in May 2007.
The reactor will be similar to North Korea’s plant at Yongbyon, which has been used to make plutonium for Kim’s nuclear bombs.
At least 350 Burmese, most of them military personnel, have received training in Russia and exiles reported that 80 others went to North Korea for instruction.
"I have to say it is childish of the Burmese generals to dream about acquiring nuclear technology since they can’t even provide regular electricity in Burma," said Thakhin Chan Tun, a former Burmese ambassador to Pyongyang, in an interview with The Irrawaddy, an exile magazine.
Than Shwe, the elderly junta leader, is not playing, however. A 37-page report leaked to Radio Free Asia reveals details of a visit to North Korea last November by a military delegation. It was led by the third man in his regime, Thura Shwe Mann, the army chief of staff.
The report describes how the 17-man Burmese group agreed to co-operate on training, special forces operations and "the building of tunnels for aircraft and ships as well as other underground military installations".
The alliance has also helped to fortify Than Shwe’s political will. This weekend he turned down a personal appeal from Ban Ki-moon, the United Nations secretary-general, to free all political prisoners including Aung San Suu Kyi, the opposition leader.
North Korea launched seven short-range ballistic missiles yesterday in defiance of United Nations resolutions. They travelled for about 250 miles before falling into the sea.
While rife government corruption and uneven development in Burma yesterday awarded Burma a spot at the bottom of Foreign Policy magazine’s Failed States Index, billions of US dollars are now known to have been channeled by the Burmese government into building the tunnels.
DVB has been tracking the development of the tunnels and underground installations in Burma for a number of years. This is the first in a series of DVB stories revealing the secretive tunnel project.
Evidence has been obtained that shows between 600 and 800 tunnels in various stages of construction, with work on some sections dating as far back as 1996.
Photographs of a number of tunnel sites clearly show North Korean advisers present. In one photograph of a work site at Pyinmanar Taung Nyo, dated 29 May 2006, North Korean advisers are seen training Burmese soldiers and technicians in tunnel construction.
Several government budget files also show evidence of foreign aid and loans being used to fund construction work.
A number of senior Burmese officials have been dismissed in recent days following the first publication of DVB’s tunnel photographs in the Yale Global Online on 8 June.
The military government has launched an investigation into how details of such a sensitive project were leaked, with associates of former intelligence chief Lieutenant General Khin Nyunt being questioned by police.
Further intelligence documents obtained by DVB show that the tunnel system is being disguised by the government as a fibre optic cable installation project.
Leaked engineering designs show, however, that some sections of the tunnels are wide enough to allow trucks to enter and leave. There is also storage space for food and weaponry, and separate rooms that would hold around 600 personnel for several months.
The documents also reveal plans to hold large rockets and satellite communication command centers inside the tunnels.
Although the financially weak Burmese government is thought to allocate some 40 per cent of its budget for military purposes, the tunnel project over the course of 13 years has likely run into the billions.
Some observers have speculated that the abrupt hike in fuel prices that sparked the September 2007 protests may have been a prelude to securing extra capital for the project.
Likewise, Burma struck a deal with China in April this year to siphon its vast offshore natural gas reserves to China’s energy hungry population, a venture that will have given the tunnel project an important boost.
Speculation that Burma is trading in military hardware with North Korea was reinforced on Monday with reports that a North Korean freighter ship believed to be carrying arms was headed in the direction of Burma.
Despite only reestablishing diplomatic ties in 2007, following North Korea’s bombing of a South Korean delegation in Rangoon in 1983, the two countries share characteristics that make them obvious allies.
According to journalist and expert on North Korea-Burma relations, Bertil Lintner, both countries have "absolutely no interest" in supporting respective UN arms embargoes.
Indeed, North Korea is one of the few countries willing to continue military trade with the pariah state, with "even China…reluctant to sell certain types of equipment to Burma", according to Lintner.
Perhaps most worryingly for countries outside of Burma’s friendship group, it has renewed an alliance with a country that is rapidly becoming the icon of a new generation of ‘rogue states’ threatening nuclear warfare.
With this in mind, speculation will likely start to circulate as to whether the tunnel network could be linked to rumours that Burma is mining uranium ore, a key ingredient for nuclear fission. No evidence has yet appeared to verify this, however.
In our next story we will reveal the purpose of these tunnels, foreign involvement in the project and what is inside the tunnels.