More than 200,000 people have been displaced by fighting in Myanmar after an alliance of ethnic minority groups launched an offensive against the military last month, the United Nations said on Wednesday.

Fighting has raged since 27 October across northern Shan state near the Chinese border after the Myanmar National Democratic Alliance Army (MNDAA), Ta'ang National Liberation Army (TNLA) and Arakan Army (AA) launched attacks on the military.

The alliance has blocked vital trade routes to China and seized a border hub in what analysts say is the biggest military challenge to the junta since it seized power in 2021.

The United Nations Office for the Coordination of Humanitarian Affairs (UNOCHA) said that as of Wednesday, "more than 200,000 people" across Shan, Chin, Kayah and Mon states and Sagaing region have been "forcibly displaced due to the fighting".

At least 75 civilians including children have been killed and 94 people wounded in the fighting, UNOCHA said, citing initial reports from the field.

Both sides have set up checkpoints on roads they control in Shan state and mobile communication remains patchy outside the main city of Lashio, hampering the delivery of aid, the UN said.

The junta has imposed martial law on several townships in the state, further hampering relief efforts, it added.

The remoteness of the rugged, jungle-clad region -- home to pipelines that supply oil and gas to China -- and patchy communications make it difficult to verify casualty numbers.

The junta has admitted it has lost ground but dismissed claims by the alliance to have seized towns across northern Shan state as "propaganda".

This week the AA launched fresh attacks on the military in western Rakhine state, shattering a fragile ceasefire that had held in the state.

In Kayah state on the Thai border, anti-junta fighters said they were battling the military near state capital Loikaw.

- 'The ground shaking' -

A car mechanic told AFP he had spent days on the road after fleeing the town of Laukkai near the China border as MNDAA fighters closed in.

"I have no experience like this in my life," the 31-year-old said by phone from Mongyang in Shan state, where he said he and hundreds of others were sheltering after escaping.

"We heard the sound of artillery as we queued to get out of that place. On the night of 7 or 8 November there were airstrikes, we even felt the ground shaking."

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" that have sprung up since the coup to fight the military's 2021 coup and its bloody crackdown on dissent.

AFP