HONG KONG — Four farmers in southwest China were killed Friday when one or more bombs released from a warplane from Myanmar struck a sugar cane field, as fighting between Myanmar’s government forces and an ethnic group pressed against the frontier with China.

The deaths, reported by Chinese state-run television, appeared to be the worst instance so far of the recently renewed conflict between Myanmar’s army and armed supporters of the Kokang people, an ethnic Chinese minority, rippling into neighboring Yunnan Province in China. Another nine people were wounded by the bombing, said Xinhua, the main Chinese state news agency.

Late on Friday, a Chinese vice minister for foreign affairs, Liu Zhenmin, summoned Myanmar’s ambassador in Beijing, U Thit Linn Ohn, to denounce the deaths and the intrusion into Chinese territory, Xinhua reported. The reports did not say whether the Chinese government thought the intrusion was deliberate or accidental.

“We urge the Myanmar side to thoroughly investigate this incident and report the findings to the Chinese side, and to sternly punish the perpetrator,” Mr. Liu told the ambassador, according to Xinhua.