At least 30,000 people have fled fighting in Myanmar's northern Shan State, the UN said on Monday.
In addition to the 20,000 who have fled across the border to China, more than 10,000 migrant workers are estimated to have returned to other parts of the country, according to humanitarian organizations quoted by the UN Office for the Coordination of Humanitarian Affairs (OCHA).
On March 6, fighting erupted between an armed insurgent group and the Myanmar military. According to state media reports, dozens have since been killed in the clashes.
Calls from the Chinese government for an immediate ceasefire were ignored. In 2015, fighting in Shan State spilled over the border into China.
Myanmar has been beset by an armed conflict since its independence from British colonial rule in 1948.
The civilian government of the country's de-facto leader Aung San Suu Kyi launched a peace process in August, but the conflict has since worsened.
A peace conference meeting scheduled for February was postponed.