The US ambassador to Myanmar has said he is "deeply concerned" about civilian deaths in a recent attack on a rebel stronghold in the country's north during the escalation of a bitter civil war.

Derek Mitchell called for both Myanmar's army and ethnic minority fighters to "stop the violence" in Kachin state near the border with China, but acknowledged that "mistrust is deep" in the increasingly bloody unrest.

"We strongly oppose any actions that harm or threaten to harm civilians," Mitchell said in comments tweeted by the United States embassy in Yangon on Thursday.

He added that he was "deeply concerned about recent civilian deaths in Laiza", the busy border town that also acts as the headquarters for the Kachin Independence Army.

Three people, including a teenager and elderly man, were killed and several others were wounded after blasts in the centre of Laiza that the rebels say were a result of army shelling. Myanmar has denied the accusations.

An upsurge in fighting since December has overshadowed Myanmar's wider political reforms, with the military's use of air strikes sparking a growing international outcry.

Britain this week joined the US and UN in criticising the military's use of jets and helicopters in the conflict, which erupted again in June 2011 when a 17-year ceasefire collapsed.

Myanmar has reached tentative peace deals with a number of major ethnic groups as part of reforms under a quasi-civilian regime that replaced outright military rule in 2011, but negotiations with the Kachin have failed to bear fruit.

Mitchell called on the two sides to "build trust, create conditions for dialogue and make peace".

"(The) US has long had a humanitarian interest in getting access to and assisting civilians caught up in the crossfire of conflict," he added.

Tens of thousands of people have been displaced in Kachin state since the breakdown of the ceasefire, with local relief workers estimating some 15,000 have taken shelter in Laiza.

The fighting has seen several shells exploding on the Chinese side of the border, with Beijing on Thursday complaining of a fresh blast two days earlier.

"We express strong concern and dissatisfaction with the relevant incident, and request that Myanmar conduct a thorough investigation and take necessary measures to prevent similar events from happening again," said China's foreign ministry spokesman Hong Lei.

No one was killed in the incident and Beijing said it had taken unspecified measures to "to preserve the management of public order" in the area.