Video published on the internet purportedly from Homs appeared to show intermittent shelling and black smoke.
International mediator Kofi Annan expressed concern about the shelling in Homs and reports of an attack on the town of al-Haffa, in Latakia province.
His spokesman said civilians were trapped in both places.
Mr Annan had demanded that UN military observers be allowed immediate entry to al-Haffa, Ahmad Fawzi said in a statement.
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.
An activist website, the Violations Documenting Centre (VDC), said there had been 29 deaths in the past week from bombardment in al-Haffa. All but three of the dead were civilians, the VDC said.
British Foreign Secretary William Hague condemned the violence in Syria. Recent massacres, such as the one in Houla in which 108 people were killed, showed the need for further action, he told the House of Commons.
“By branding their opponents terrorists and using tanks against them, the regime is driving Syrians to take up arms to defend their homes and by singling out particular communities it is inflaming sectarian tension,” Mr Hague said.
The Syrian government blames the violence on foreign-backed armed terrorist gangs.
UK foreign secretary William Hague believes al-Qaeda linked terrorists are operating in Syria
Separately, human rights activists said Syrian government forces bombarded the town of Rastan, in central Homs province.
Government troops used helicopters to strafe rebel positions in Rastan, which has been under intermittent army shelling “for months,” said the London-based Syrian Observatory for Human Rights.
“The regime is now using helicopters more after its ground troops suffered major losses,” the head of the Observatory, Rami Abdul-Rahman, told the Associated Press news agency.
UN monitors saw Syrian helicopters firing on Rastan and another rebel stronghold, Talbisa, a UN spokeswoman said in a statement quoted by Reuters.
“UN observers reported heavy fighting in Rastan and Talbisa, north of the city [of Homs], with mortar shelling as well as firing from helicopters, machine guns and smaller arms,” spokeswoman Sausan Ghosheh said in a statement.
These reports cannot be confirmed independently because Syria heavily restricts journalists’ freedom of movement.
Source: BBC News