Hong Lei, a spokesman for China's Foreign Ministry, said a bomb landed inside China Tuesday, less than a mile from the border with Myanmar, in the fighting between the Myanmar military and the Kachin Independence Arm, Xinhua reported Thursday.
"China has lodged urgent representation to Myanmar over the incident, to express grave concerns and dissatisfaction," Hong said.
No one was killed or injured by the bomb, the fourth to be dropped inside China because of the conflict in Myanmar, formerly known as Burma.
China called on Myanmar leaders to investigate the matter and work to ensure similar incidents don't occur again.
Hong urged opposing sides to exercise the "utmost restraint," resolve their conflicts through negotiations and maintain stability near the countries' common border.
"China has for a long time closely followed the development of the situation in northern Myanmar and the influence on the border area," he said.
Chinese news organizations reported that ethnic Kachin refugees have fled to China to escape the violence. Reports said the number of refugees from Myanmar was growing but they did not provide estimates, The New York Times reported.
Officials in Yingjiang county in Yunnan province were establishing a refugee camp with running water and electricity in case refugees arrive with nowhere else to live.
Since a 17-year cease-fire collapsed in June 2011, Myanmar forces have fought the Kachin Independence Army, which controlled a large territory in Kachin state. The army began a new offensive in mid-December that observers say may involve heavy artillery, attack helicopters and other aircraft to overpower guerrilla positions around Laiza, the rebels' main administrative center.