BEIJING (REUTERS) - China on Wednesday said it had lodged a protest with Myanmar after shelling injured five people in the southwestern province of Yunnan last week, the latest incident of cross-border fire that has strained ties between the neighbours.

Myanmar's parliament has extended martial law for three months in a region along its border with China that is rife with clashes between Myanmar government troops and a rebel group.

About 60,000 people had crossed the border into China due to the clashes, the United Nations has estimated, although some started returning last month.

Chinese foreign ministry spokesman Hong Lei said Myanmar had told China it was investigating last week's incident, and Beijing awaits the outcome.

"China has lodged solemn representations on the injuries caused to Chinese people on the border from the recent shelling by Myanmar," Hong said.

China had asked Myanmar for a "responsible explanation" for the incident and urged it to "take effective measures" to prevent such incidents, Hong said.

China was infuriated in March when five people were killed by stray bombs falling into Yunnan during the fighting in Myanmar, which has intensified in the past few months.

Last Thursday, shells landed in an area near the small city of Lincang. Four houses and three vehicles were also damaged.

China has repeatedly demanded effective action from Myanmar to contain the fighting.

Myanmar government soldiers have been battling rebels who were dug in as close as 500 metres from the border area, Myanmar's Information Ministry said in April.