Thousands of people are feared to have been displaced by four days of fighting in northern Myanmar, including several hundred who reportedly fled to China, the United Nations said on Monday.

On Friday, three powerful ethnic armed organisations attacked junta troops across a swathe of northern Shan state, home to a planned billion-dollar rail link, part of Beijing's Belt and Road infrastructure project.

The Ta'ang National Liberation Army (TNLA), the Arakan Army (AA) and the Myanmar National Democratic Alliance Army (MNDAA) have since said they have seized several military posts and key roads.

The three groups -- which analysts say can call on at least 15,000 fighters between them -- have fought regularly with the military over autonomy and control of resources.

"As of 30 October, over 6,200 individuals have reportedly been newly displaced" by the fighting, the UN Office for the Coordination of Humanitarian Affairs said in a statement.

"Additionally, several hundred people have reportedly crossed the border into China in search of safety."

It said it expected more people to be displaced amid ongoing clashes.

One man in the town of Hsenwi -- which sits on a vital road to China -- told AFP on Monday that he was sheltering with around 100 others in a Buddhist monastery.

"We did not have a place to escape," said the man, who requested anonymity for security reasons.

"We tried to leave but we could not drive out of the town as there was ongoing fighting."

China's foreign ministry said Friday it was "closely following" the fighting and called on all sides to prevent the situation from escalating.

Myriad ethnic armed groups operate in Myanmar's frontier areas and have for decades clashed with the government.

Since the military seized power in 2021, several of the ethnic groups have allied with People's Defence Forces that have sprung up to battle the junta.

More than 1.6 million people have been displaced by the violence that has flared since the military ousted Aung San Suu Kyi's civilian government, according to the United Nations' estimate.

AFP