BEIJING (Reuters) - China's Defence Ministry said on Thursday that it investigating whether more stray bombs had fallen in Chinese territory during fighting between Myanmar's government and rebels, after five people were killed earlier this month.

Those deaths, in the south-western Chinese province of Yunnan, infuriated Beijing, which promised a "decisive" response if there was a repetition.

Pictures have since surfaced on Chinese websites which appear to show more bombs from Myanmar fell in Yunnan last week. "At present we are organising a verification process to look into this incident," Defence Ministry spokesman Geng Yansheng told a regular monthly news briefing.

A joint Chinese and Myanmar has just finished a three-day probe into the bombing which killed five, he added, without giving details.

Tens of thousands of people, many of them ethnic Chinese, have fled the fighting in northeastern Myanmar's Kokang region into China.

On Wednesday evening, two grenades were fired into a Myanmar military base in Lashio, the largest city in Shan state, located some 130km from the Kokang border region where the military is battling at least three ethnic rebel militant groups, said an officer with the Lashio police. "There are no reports of casualties," said the officer who asked not to be named, as he was not authorised to speak to media.

Myanmar has said Chinese mercenaries were fighting with the rebels, and it has urged China to cooperate to prevent "terrorist attacks" being launched from Chinese territory.

China has denied that any attacks into Myanmar have been launched from its territory.

Geng denied that any serving Chinese soldiers were fighting with the rebels, adding that China will not allow anyone to use Chinese territory to damage relations or stability along the border.

The main rebel group is called the Myanmar National Democratic Alliance Army (MNDAA), which is led by ethnic Chinese commander Peng Jiasheng.

The MNDAA was formed from remnants of the Communist Party of Burma, a powerful China-backed guerrilla force that battled the Myanmar government until it splintered in 1989.

The group struck a truce with the government which lasted until 2009, when government troops took over their region in a conflict that pushed tens of thousands of refugees into China's Yunnan province.

China and Myanmar share a 2,000km border, much of it remote and hard to access.