Beijing on Tuesday confirmed there had been Chinese casualties after ethnic armed groups fighting Myanmar's junta seized outposts in the country's north along the border with China.
China's foreign ministry spokesperson Wang Wenbin did not say whether the Chinese were killed or wounded, or where precisely the incident had taken place.
Local media in Myanmar reported on Saturday that one Chinese person was killed and another two wounded after the military shelled the town of Laiza, home to the headquarters of ethnic armed group the Kachin Independence Army.
The reports said one shell had landed on the China side of the border.
On Tuesday, Wang said Beijing "expresses strong dissatisfaction with the escalation of the armed conflict and the casualties caused to Chinese personnel".
China, he said, "has lodged solemn protest with relevant parties" and is "paying close attention to the conflict situation in northern Myanmar".
"China once again demands that all parties involved in the conflict in northern Myanmar immediately cease fire," Wang said.
They must "take realistic measures to prevent any incidents that endanger the lives and property of people in China's border areas from happening again", he said.
Beijing, he said, would take "necessary measures to safeguard the lives and property of its citizens."
China has since last week expressed its concern over the escalating fighting near its border with Myanmar.
Senior Chinese diplomat Nong Rong visited the country from Friday to Sunday and met with senior junta officials over the weekend, urging Myanmar to "cooperate" in maintaining stability on their shared border.
He also urged the junta to "earnestly ensure the safety of the lives and property of Chinese border area residents, and take effective measures to strengthen the security of Chinese personnel, institutions and projects in Myanmar", Beijing's foreign ministry said.
Belt and Road
Since launching a joint offensive the Myanmar National Democratic Alliance Army (MNDAA), the Ta'ang National Liberation Army (TNLA) and the Arakan Army (AA) say they have captured dozens of military outposts in northern Shan state.
The region is home to pipelines that transport oil and gas to China and a planned billion-dollar rail link that is part of Beijing's Belt and Road infrastructure initiative.
The ethnic armed groups have previously said they would not allow China-backed projects to be targeted.
TNLA fighters were battling junta troops for control of the town of Namhkam on Tuesday, a TNLA spokesman told AFP, without giving details on casualties.
Since Saturday Myanmar's military has not responded to repeated requests for comment on the progress of the fighting in northern Shan state.
In the trade hub of Muse, around 25 kilometres from Namhkam, internet and phone connections were mostly down, a resident told AFP, requesting anonymity for security reasons.
"Border trade is completely stopped," they said.
"We've lost contact with Namkham for a long time. There have been fighting in other towns near our Muse.
"We do not know when it will be our turn. People are living in fear," they said.