The UN human rights office urged the international community to take immediate steps to halt the spiral of violence in Myanmar.

Myanmar's military seized power on February 1 last year, ousting the civilian government and arresting its de facto leader, Aung San Suu Kyi. The junta has since waged a bloody crackdown on dissent.

In a report covering the period since the takeover, UN human rights chief Michelle Bachelet's office said Myanmar's military and security forces had shown a flagrant disregard for human life.

It described how the military had bombarded populated areas with air strikes and heavy weapons and deliberately targeted civilians, many of whom were shot in the head, burned to death, arbitrarily arrested, tortured or used as human shields.

"We have really been able to identify a pattern over the past year, which indicates that this is planned, coordinated, systemic attacks; that there are clear indications that they would amount to war crimes and crimes against humanity," UN human rights office spokeswoman Ravina Shamdasani told reporters in Geneva.

"This is the clearest yet indication of the commission of these crimes."

The report said that at least 1,600 people had been killed by security forces and their affiliates, while more than 12,500 people had been detained.

At least 440,000 others have been displaced and 14 million are in need of urgent humanitarian assistance, the delivery of which has largely been blocked by the military in new and pre-existing areas of need, said the report.

"Meaningful action by the international community is urgently needed to stop yet more individuals from being stripped of their rights, their lives and their livelihoods," Bachelet said.

"The appalling breadth and scale of violations of international law suffered by the people of Myanmar demand a firm, unified, and resolute international response."

The report was released for the ongoing 49th regular session of the UN Human Rights Council, which runs until April 1.

Bachelet is due to present the report to the council on March 21.