The resumption of heavy shelling comes after a few days of relative calm, during which UN humanitarian chief Valerie Amos visited Homs and said part of the city had been completely destroyed.
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"Thirty tanks entered my neighborhood at seven this morning and they are using their cannons to fire on houses," said Karam Abu Rabea, a resident of the Karm al-Zeitoun district in Homs.
"There is gunfire and they are using rocket-propelled grenades," he said, adding that the streets had emptied and people were taking refuge in their homes.
In other parts of the country, Assad's opponents took to the streets to demonstrate after Friday Muslim prayers.
An activist in the Khalidiya district of Homs, also a centre of opposition to Assad's rule, said he had heard mortars landing in his neighborhood since the early hours of the morning and the distant buzz of a spotter plane.
The UK-based Syrian Observatory for Human Rights added that security forces had killed two people in house-to-house raids near the Turkish border and three were killed in Hama city.
Humanitarian chief Amos said on Thursday she had asked the Syrian government to allow access for humanitarian aid to victims of the turmoil.
UN-Arab League envoy Kofi Annan is due to arrive in Damascus on Saturday to try to calm a year-old conflict which threatens to turn into civil war.
Meanwhile, Ankara's media reported that two Syrian generals and a colonel have defected to Turkey.
TRT television said the three former officers were among 234 Syrians who crossed into Turkey since Thursday. Some 12,000 Syrians now live in six refugee camps in Turkey.
On Thursday, Syria's deputy oil minister became the highest-ranking civilian official to join the opposition and urged his countrymen to "abandon this sinking ship" as the nation spirals toward civil war.
Abdo Husameddine, 58, announced in a video that he has defected from President Bashar Assad's regime.
AP and Reuters contributed to this report
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