BEIJING (AFP) - Four Chinese people were killed in southwestern Yunnan province Friday by a bomb dropped from a Myanmar warplane, state media reported, days after Beijing warned of escalating violence across the border.

The official news agency Xinhua said the bomb hit a sugarcane field in Lincang city, killing the four workers and injuring nine others.

It came after China's foreign ministry said earlier this week that a house in Yunnan had been hit by shelling from across the border in Myanmar, where the military are fighting rebel forces.

Beijing has previously warned of a threat to border stability after the dramatic upsurge in ethnic conflict in the remote Kokang region in Myanmar's northeastern Shan state.

Last month, Myanmar declared a state of emergency in Kokang in response to the conflict, which began on Feb 9.

The unrest has virtually emptied the main Kokang town of Laukkai, the epicentre of the fighting, with streets in the once-bustling frontier community transformed into a battleground.

More than 30,000 people have fled from Myanmar into Yunnan province, according to Xinhua.

Early Saturday morning, the agency reported that Chinese Deputy Foreign Minister Liu Zhenmin had summoned Myanmar ambassador Thit Linn Ohn on Friday night to protest the deaths.

Liu urged Myanmar to "thoroughly investigate" the case and "take immediate and effective measures to prevent recurrence of such incidents," said Xinhua.

He called on the Myanmar authorities to "safeguard the security and stability in the border areas between China and Myanmar," the agency added.

Chinese foreign ministry spokesman Hong Lei said on Tuesday that two days previously "some shells fell on Chinese territory, damaging a civilian home".