Almost 50,000 people have been displaced by fighting in northern Myanmar after an alliance of ethnic armed groups launched an offensive against the military two weeks ago, the United Nations said Friday.

Fighting has raged across northern Shan state near the Chinese border in what analysts say poses the biggest military challenge to the junta since it seized power in 2021.

The Myanmar National Democratic Alliance Army, Ta'ang National Liberation Army and the Arakan Army have blocked vital trade routes to China and say they have captured dozens of military outposts.

"As of 9 November, almost 50,000 people in northern Shan were forced into displacement," the United Nations Office for the Coordination of Humanitarian Affairs (UNOCHA) said in an update.

There were still clashes on Friday in the town of Hsenwi, which sits on the important Muse and Chinshwehaw trade routes to China, a resident told AFP.

A resident of Shan state's Lashio township said there was no fighting in the town, home to the military's northeastern command, although the airport -- which was closed shortly after the outbreak of fighting -- remained shut.

"Town residents are going about their normal daily life although we heard some artillery shooting from outside of the town at night," she told AFP on condition of anonymity.

Internet and phone services were disrupted outside Lashio, hindering humanitarian responses to the fighting, UNOCHA said.

Restrictions on transport and availability of cash were also hampering efforts by local humanitarian groups to give out aid, it said.

A further 40,000 people have been displaced by clashes between the military and its opponents in neighbouring Sagaing region and Kachin state since early November, UNOCHA said.

The military has made little comment on the surprise offensive but the junta-appointed president warned this week that Myanmar could end up "split into various parts" if the military was unable to "manage" the fighting.

The remoteness of the rugged, jungle-clad region and patchy communications make it difficult to verify casualty numbers.

State media reported on Friday that businesses licensed to trade through Muse and Chinshwehaw would be able to move to other crossings into China and Thailand.

The decision to allow traders to move to alternative crossings had been made to ensure smooth trade, the Global New Light of Myanmar said without mentioning the clashes.

Muse and Chinshwehaw carried more than two-fifths of the $5.32 billion worth of border trade between Myanmar and its neighbours between April and November this year, according to commerce ministry figures.

Analysts say even more crosses through the black market and that around a billion dollars of that comes from natural gas piped into China through Shan state via infrastructure the ethnic armed groups have said they will not harm.

Beijing "understood" that infrastructure had not been affected by the clashes, a Chinese foreign ministry spokesman said on Friday.

Communication blackout 

China, a major junta ally and arms supplier, confirmed on Tuesday there had been Chinese casualties as a result of the clashes in Myanmar.

A foreign ministry spokesperson did not say whether the Chinese were killed or wounded, nor where precisely the incident had taken place.

Myanmar's borderlands are home to more than a dozen ethnic armed groups, some of which have fought the military for decades over autonomy and control of lucrative resources.

Some have trained and equipped newer "People's Defence Forces" (PDF) that have sprung up to fight the military's bloody crackdown on dissent since the coup.

Several PDF groups claimed this week to have seized the town of Kawlin in Sagaing region, home to mostly ethnic-majority Bamar and a traditional military recruiting ground.

AFP was unable to reach residents in the area, where internet and phone lines are largely cut.

Sagaing, which borders Shan and Kachin states, has become a resistance hotspot.

Dozens of PDF groups are active across Sagaing, where the military is accused of burning villages and massacring inhabitants.