Syrian government forces have renewed their attack on the city of Homs, one of the focal points of the uprising against President Bashar al-Assad.
Video published on the internet purportedly from Homs showed intermittent shelling and black smoke.
UN mediator Kofi Annan is concerned civilians have been trapped in Homs and al-Haffa, a town in Latakia province also said to be under attack.
The US says it fears the government may be planning "another massacre".
Mr Annan's spokesman, Ahmad Fawzi, said civilians had been trapped in both Homs and al-Haffa.
Mr Annan was demanding immediate entry to al-Haffa for UN military observers be allowed, he added.
As joint envoy for the UN and the Arab League, Mr Annan brokered a six-point peace plan, including a ceasefire which came into nominal effect two months ago but has now been virtually abandoned.
BBC Middle East bureau editor Paul Danahar, who visited Homs with a team of UN observers earlier on Monday, said the Syrian army appeared to be using an unmanned surveillance drone to select buildings as targets for shelling.
Our correspondent reported a steady stream of mortar rounds landing in the old city of Homs at a rate of about one a minute.
He says he understands that the UN team - which has been trying for two days to gain access to the old city - has still not succeeded.
All the UN can do is stand by and watch, our correspondent says.
US state department spokeswoman Victoria Nuland said it was "deeply alarmed" at "reports from inside Syria that the regime may be organising another massacre".
Such an attack could happen, it suggested, in al-Haffa or the towns of Deir el-Zour, Homs or Hama, or in the suburbs of the capital, Damascus.
According to the London-based Syrian Observatory for Human Rights, at least 74 people were killed across Syria on Monday.
An activist website, the Violations Documenting Centre, said there had been 29 deaths in the past week from bombardment in al-Haffa. All but three of the dead were civilians, it added.
These reports cannot be confirmed independently because Syria heavily restricts journalists' freedom of movement.
British Foreign Secretary William Hague condemned the violence in Syria, accusing the government of "inflaming sectarian tension".
The Syrian government blames the violence on foreign-backed armed terrorist gangs.
Separately, UN monitors and human rights activists said Syrian government forces had used helicopters to bombard the town of Rastan, in Homs province.
The town has been under intermittent army shelling "for months", the Observatory said.
UN spokeswoman Sausan Ghosheh said monitors had seen Syrian helicopters firing on Rastan and another rebel stronghold, Talbisa.
In Talbisa, rebels from the Free Syrian Army captured soldiers from government forces, she added.