The US fears the possibility of a new massacre in Haffa, Latakia, where the Assad regime had denied access to UN observers, it has been reported.
"The UN made a particular appeal to be able to get into Haffa because it had heard concerning reports about regime plans," said Victoria Nuland, US state department spokeswoman in a press conference in Washington.
"At the same time, if you follow Syrian media, they were claiming that the opposition would cause a massacre," she added.
UN- Arab League special envoy Kofi Annan and the UN Secretary General Ban Ki-moon expressed concern about the violence in Haffa.
Annan said that there were indications that a large number of civilians being trapped in Homs and Haffa.
"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," said a statement from the UN.
Meanwhile, heavy shelling and firing continued in Homs, according to a report by a BBC correspondent travelling with UN observers.
Heavy fighting and explosions reported in Rastan and Talbisa near Homs and in Idlib province. According to the London -based Syrian Observatory for Human Rights, 10 people were killed in the city of Deir al-Zour.
A political solution to the violence in Syria is still hanging in the balance; Russia, the strongest ally of Syria called for Iran's participation in ending the conflict.
Russia's Foreign Minister Sergei Lavrov is expected to travel to Tehran on Wednesday to discuss the issue which invited an outright rebuke from US Secretary of State Hillary Clinton.
She opined that such a move would only help stage-manage the repression, the BBC reported.
According to Syrian activists, 108 people were killed in Houla, Homs province and 78 people were killed in the village of Qubair, in Hama province by the Assad forces.
Earlier, Bashar al-Assad blamed foreign backed terrorist groups of indulging violence in Syria and even condemned the Houla massacre where children and women were mercilessly butchered.