The U.S. expressed fears Monday that the Syrian government may be organizing another massacre in the town of Haffa in coastal Latakia province, where U.N. military observers have been denied access.

International mediator Kofi Annan said he was gravely concerned about violence in Haffa, including reports of mortar, helicopter and tank attacks, the BBC reported. He also cited fears about Homs.

U.N. Secretary-General Ban Ki-moon is demanding Syrian authorities grant  the U.N. unimpeded access to Haffa, the Voice of America reported.  

A BBC correspondent traveling with U.N. observers witnessed sustained and heavy shelling in the old city of Homs.

The U.N. made a particular appeal to be able to get into Haffa because it had heard concerning reports about regime plans, U.S. State Department spokeswoman Victoria Nuland said in Washington. At the same time, if you follow Syrian media, they were claiming that the opposition would cause a massacre.

“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,” Nuland said.

In a statement, Annan, joint envoy for the U.N. and Arab League, said there were indications that a large number of civilians are trapped in Homs and Haffa. Syrian government helicopter gunships strafed rebel positions in Haffa, U.N. observers and human rights activists said.

U.N. cease-fire monitors reported artillery shelling and machine-gun fire in the Khaldiyeh section of Homs as well as the towns of Rastan and Talbiseh, to the north, The New York Times reported. The monitors also reported the military’s use of helicopter gunships -- a relatively new tactic employed by the Syrian Army, first observed by anti-government activists in attacks on armed rebels around the major port of Latakia a week ago. The helicopter attacks are regarded as a significant escalation by the government.