China has summoned the Myanmar ambassador for a meeting in Beijing after a bomb from a Myanmar warplane fell in Chinese territory, killing four people in a sugarcane field, according to China’s foreign ministry.

The incident occurred on Friday as Myanmar government forces have ramped up their battle in recent weeks against ethnic Chinese rebels in the country's Kokang region bordering China.

China has warned Myanmar to "lower the temperature," and the fighting has forced thousands of people, many of them ethnic Chinese, to flee from Myanmar into China’s southwestern Yunnan province.

Newspapers in Myanmar reported that government forces launched airstrikes against the rebels and heavy clashes took place near the border.

According to China’s official Xinhua News Agency, the bomb hit a field in the Chinese city of Lincang in Yunnan province. State media reported that nine people were also wounded. 

Al Jazeera's Florence Looi, reporting from Yangon, said Kokang region has been under a state of emergency since mid-February and that there are few independent reports coming out from the area.

"This appears to be the worst spillover of violence into China since fighting began in the Kokang region more than a month ago," she said.

The incident comes within a week after a stray shell from Myanmar destroyed a house in Chinese territory, prompting condemnation from Beijing.

"These two latest incidents suggest that there is very heavy fighting going on in that area," said Looi.

A Chinese vice foreign minister, Liu Zhenmin, has urged the Myanmar ambassador, Thit Linn Ohn, to investigate Friday's incident and ensure the safety of the border area.

The ministry said in a statement that they want Myanmar to report all investigation findings to China and to punish the culprits.

Border patrol 

On Saturday, according to a statement on China's Defence Ministry’s website, Shen Jinke, a spokesman for China’s air force, said China had sent warplanes to patrol its border with Myanmar and to ward off any Myanmar planes close to the border. 

China will continue to monitor the airspace along the border, Shen said. The countries share a 2,000km border. China, which disavows links with Myanmar’s ethnic Chinese rebels, has denied that any attacks into Myanmar have been launched from its territory.

Myanmar officials have stated that ex-Chinese soldiers have trained these rebels, a claim denied by the fighters. 

The rebels are from a group called the Myanmar National Democratic Alliance Army whose leader, Peng Jiasheng, is ethnic Chinese.

Government officials say the upswing in fighting is due to the rebels' attempt to seize the capital of the self-administered Kokang region, Laukkai.

The rebel group was formed out the remnants of the Communist Part of Burma, which was backed by China until it signed a ceasefire agreement with the Myanmar government in 1989. The agreement lasted until 2009, when Myanmar government troops took over their region in a conflict that saw tens of thousands flee into China.

Source: Agencies