Fighting between Burma's military forces and an ethnic militia in northern Burma has forced hundreds of civilians to flee across the border to China.

A spokesman for the Kachin rebels said from Thailand that Burmese troops pushed the ethnic forces from a key mountain post Tuesday. Kachin rebels destroyed a strategic bridge used by the government trucks.

Campaign for Burma, a Washington-based advocacy group, said that four Kachin fighters and at least 16 Burmese soldiers were killed in the clashes. The group said about 2,000 Burmese villagers have fled their homes, most of them crossing the border into China.

Government troops have clashed for several days with forces controlled by Burma's Kachin minority in a strategic region where China is building hydropower plants. The fighting erupted Thursday in the township of Momauk near the Chinese border.

The last 30 of some 100 Chinese engineers working on two hydropower plants and dams were also permitted to leave Tuesday, bringing the operations to a standstill.

Burma's officials have not made any statements regarding the clashes. It is almost impossible for outsiders to confirm reports about the fighting. The border areas are largely off limits to foreigners and journalists.

The Kachin Independence Army, like several other ethnic militias in Burma, signed a cease-fire agreement with the central government several years ago. But those agreements began to break down in 2009, when Burma demanded that the militia groups come under central authority and serve as part of a national border guard. That prompted some militias to resume fighting.

Last year, Burma held its first election in 20 years, in what the former ruling military council called a step toward full civilian control. However, rights activists and several governments, including the United States, say the election was flawed and solidified military rule.

A number of countries have imposed sanctions on Burma's government because of its human rights record and lack of progress in political reforms.