The United States on Monday voiced concern that Syria's government is planning a new massacre after accounts that tanks have approached the edge of the city of Al-Heffa.

A spokesman for former UN secretary general Kofi Annan, who drafted a fledgling UN-Arab League peace plan, earlier warned that civilians were trapped in the northwestern city and demanded the immediate entry of UN monitors.

"The United States joins joint special envoy Kofi Annan in expressing deep alarm by reports from inside Syria that the regime may be organizing another massacre," State Department spokeswoman Victoria Nuland told reporters.

"We remind Syrian commanders of one of the lessons from Bosnia: The international community can and does learn what units were responsible for crimes against humanity and you will be held responsible for your actions," she said.

Syria has already experienced two major massacres recently, with 55 people killed last week in Al-Kubeir and at least 108, about half of them children, killed in Houla on May 25.

Nuland said that the tactics showed that the regime of President Bashar al-Assad, cracking down on the most severe threat to his family's four-decade rule, was "increasingly desperate."

"What government voluntarily uses helicopters and fires from them on their own civilians if they're not desperate? What government depends on a bunch of thugs in trucks -- irregulars -- if they're not desperate?" she said.

But Nuland again ruled out US military intervention. Russia has strongly opposed the prospect of force to remove Assad, whose family has been a key ally of Moscow since the Cold War.

"The concern has been that putting foreign military forces into this situation -- which is on the verge, as everybody has said, of becoming a civil war -- will turn it into a proxy war," Nuland said.

[A handout picture released by Shaam News Network allegedly shows Syrian army tanks hidden inside a schoolyard in April 2012. AFP Photo]