A Burmese ethnic militia group, the Kachin Independence Army, is in a tense stand-off with the army following the arrest of three of its members.
It says one of its offices has been surrounded by the Burmese military.
The KIA also said that its forces have themselves surrounded an army camp near the Chinese border, the BBC Burmese service has reported.
It has had a 16-year ceasefire with the military government and seeks autonomy for the Kachin people.
The group has refused to sign a deal with the government to disarm and become an official border guard force.
The KIA and its civilian organisation have been allowed to control a large swathe of northern Burma as part of a ceasefire agreement with the country's ruling generals.
They provide power, roads and schools funded by taxes on the brisk trade from China as well as the jade and gold mines and teak.
But the government's deadline to disband had prompted fears of conflict with those groups, such as the KIA, who refused to do so.
The BBC's Alastair Leithead visited the KIA in Laiza, northern Burma, earlier this year, where KIA generals said they were preparing for the worst.
"I can't say if there will be war for sure, but the government wants us to become a border guard force for them by the end of the month," said the KIA's Chief of Staff, Maj Gen Gam Shawng, in February.
"We will not do that, or disarm, until they have given us a place in a federal union and ethnic rights as was agreed in 1947."
Before the latest reports, the situation appeared to have calmed.
Our correspondent says the risk of violence between the Burmese army and both the KIA and the United Wa State army, had appeared to have been reduced partly due to pressure from neighbouring China.
There are around two dozen ethnic groups in Burma, mostly scattered around its borders, and the biggest have been in various states of ceasefire or civil war over the past few decades.
The KIA is one of the biggest - their commanders say it includes 10,000 regular troops and 10,000 reservists, but it is impossible to know for sure.
The Burmese army is one of the biggest in Asia.