Syria conflict: Fears of new massacre in Haffa
The US has expressed fears that the Syrian government "may be organising another massacre" in the town of Haffa in Latakia province, where UN military observers have been denied access.
UN Secretary General Ban Ki-moon said it was vital that the observers should be allowed into the town.
He and peace envoy Kofi Annan expressed grave concern about violence there.
A BBC correspondent travelling with UN observers witnessed sustained and heavy shelling in the old city of Homs.
The UN observers are in Syria to monitor the implementation of a putative ceasefire, brokered by Mr Annan, which has all but collapsed.
"The UN made a particular appeal to be able to get into Haffa because it had heard concerning reports about regime plans," US state department spokeswoman Victoria Nuland told a news briefing in Washington.
"At the same time, if you follow Syrian media, they were claiming that the opposition would cause a massacre."
Earlier this month, activists said Syrian government forces killed 108 people in the region of Houla, in Homs province, and 78 people in the village of Qubair, in Hama province.
But Ms Nuland downplayed the idea of foreign intervention in Syria, saying the emphasis would remain on using political and economic measures to pressure the government of President Bashar al-Assad.
The Syrian government blames the violence on foreign-backed armed terrorist gangs.
In a statement, Mr Annan, joint envoy for the UN and Arab League, said there were "indications that a large number of civilians are trapped" in Homs and Haffa.
Later Mr Ban spoke of the "great danger" to civilians under fire.
"The secretary general underlines the importance of unimpeded access by UNSMIS to Haffa, amid reports of a build-up of government forces around the town," he said in a statement.
Syrian government helicopter gunships strafed rebel positions in Haffa, UN observers and human rights activists said.
Heavy fighting was also reported in Rastan and Talbisa, north of Homs city, "with artillery and mortar shelling, as well as firing from helicopters, machine guns and smaller arms", said a statement from the UN Supervision Mission in Syria (Unsmis).
A car bomb exploded in the city of Deir al-Zour, killing 10 people, the London-based Syrian Observatory for Human Rights said, quoting sources on the ground.
Fighting was also reported in Idlib province.
It is impossible to confirm these reports independently, as Syria heavily restricts journalists' freedom of movement.
Russia, which supports the Syrian government, called for Iran to take a role in ending the 15-month old conflict.
Last week, US Secretary of State Hillary Clinton rejected the idea of involving Iran, which she accused of "helping to stage-manage the repression" in Syria.
Russian Foreign Minister Sergei Lavrov will travel to Tehran on Wednesday to discuss the initiative, the foreign ministry said.