Beijing on Monday reiterated that Burma was to blame for the deaths of five Chinese people across in Yunnan province, despite Naypyidaw expressing “deep sorrow” for the bombing.

China said a bomb fell from a Burmese aircraft on a sugarcane field in Yunnan on Friday, killing four people and wounding nine. One of the injured later died.

On Saturday, a senior officer said China’s military would take “decisive measures” if there was a repeat attack by Burmese forces on its territory. Beijing also summoned Burma’s ambassador to register a protest.

Speaking at his annual news conference on Sunday, Chinese Premier Li Keqiang said that the government was able to “firmly defend” the stability of the border.

“Our government, the Foreign Ministry and the military have all lodged a severe protest to the Burma side. We have the responsibility and ability to firmly defend the stability of the China-Burma border and firmly protect the lives, property and security of China’s people,” Li told reporters.

Burma has said the bomb may have been lobbed by rebels it is fighting in the Kokang region bordering China.

Burma said they would like to express their deep sorrow for the deaths and injuries of Chinese nationals living in the border areas, the government said in a statement published in a local newspaper.

It added that the two countries’ foreign and defence ministries were in direct contact over an investigation into the incident.

China’s foreign ministry however maintained resolute that the deaths were caused by a Burma airforce plane.

“The facts are clear that a bomb from a Burma military plane caused the death of Chinese people. As China requested, Burma already dispatched a working group to the China-Burma border area to conduct a joint investigation with China.

This investigation has begun,” Foreign Ministry spokesman Hong Lei told a daily news briefing.

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“We once again urge the relevant parties in the Burma conflict to treat China’s concerns seriously, earnestly maintain restraint, quickly pacify the situation and recover peace and stability in the northern region of Burma,” Hong added.

The deaths have caused considerable anger in China, with some people taking to social media sites urging a tough response.

However, the Global Times, an influential tabloid published by the official People’s Daily, said in an editorial that the best way for China to ensure stability along its border was to actively push for peace in northern Burma.

The rebels are from a group called the Myanmar National Democratic Alliance Army (MNDAA), which is led by ethnic Chinese commander Peng Jiasheng.

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

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