An assault by the Syrian army killed more than 20 people in the increasingly fractured city of Homs on Monday, according to activists who decried what they called a five-day assault on civilians.

Videos uploaded Thursday onto YouTube showed a city at war, with the frequent thump of heavy artillery and rattle of gunfire down alleyways that people in videos identified as the Baba Amr suburb of Syria’s third-largest city.

The rest of the city was locked down by a heavy security presence, said an activist in Homs who goes by the nickname Abu Rami, with people allowed to walk only one or two streets away from their homes without being arrested or shot.

All shops and markets were closed, he said, and like the rest of the country Homs is short of fuel, with people burning wood for warmth, and of food. Reached by Skype, Abu Rami described civilians being injured by nail bombs and houses destroyed by shells, killing entire families.

The surge in violence, which followed more than 100 deaths the previous week in fighting in the northwestern province of Idlib, comes as a delegation from the Arab League is gathered in Damascus preparing to start work on a mission to monitor whether the Syrian government is keeping to an agreement signed this month.

That agreement calls for the withdrawal of troops from urban areas, where the forces of President Bashar al-Assad’s government have been endeavoring to control a nine-month uprising that is becoming increasingly violent.

The most organized opposition coalition outside the country, the Syrian National Council, called on Sunday for the observers to go to Homs immediately, “specifically to the besieged neighborhoods, to fulfill their stated mission.”

Echoing the views of opposition groups inside the country, they said that the observation mission should begin its work in earnest or leave.

More than 100 people have been killed in Homs in the past four days, Abu Rami said. However, he added that he believed that an increasing number of soldiers had defected to join loose armed opposition groups, indicating that the security forces are probably confronting rebel gunmen as well as unarmed civilian dissidents.

As the Syrian government controls media access tightly, it was not possible to verify the account independently.