Syrian troops have pushed ahead with a new assault on the northern region of Idlib, shelling one of the centres of the uprising against President Bashar al-Assad's rule and sending families fleeing for safety as members of the armed oppostion tried to fend off the attack.

Abu Hani, an activist in Idlib, told Al Jazeera the attack began early on Saturday morning, with tanks and artillery shelling buildings at two minute intervals from all sides of the city.

Troops killed 16 opposition fighters in an ambush in Idlib province on Saturday while the opposition forces killed four soldiers and captured five, the Syrian Observatory for Human Rights said.

Nationwide, 31 people were killed, the UK-based watchdog said.

The military operation has raised fears that the government is planning a new all-out offensive in Idlib like the bloody siege last month that captured a restive part of the city of Homs, further south.

While the fighting raged, UN and Arab League envoy Kofi Annan met with Assad in Damascus during a high-profile international mission to mediate an end to the country's year-long conflict.

Al Jazeera's Sue Turton, reporting from Beirut, said the shelling of Idlib occured just "a few hours after Kofi Annan touched down in Damascus".

"The bombardment carried on for hours and from what we understand it is still continuing with tanks trying to push into the city reinforced by foot soldiers," she said.

"We are now hearing of fierce fighting between the government troops and the Free Syrian Army."

Ambulances

Armed opposition fighters dashed through the streets of Idlib, taking cover behind the corners of buildings in clashes with the troops.


Free Syria Army's Riad Al Asaad and Robert Baer, a former CIA case officer on the violence in Idlib

Wounded fighters were pulled into trucks to be sped to clinics for treatment.

A group of men used shovels to destroy speed bumps along one street to allow ambulances and other rescue vehicles to drive faster.

Military reinforcements have been pouring into Idlib this week, including dozens of tanks and armoured personnel carriers.

The moves suggested the government was now turning its focus on Idlib after recapturing the rebel-held district of Bab Amr in the central city of Homs, in a month-long assault that reportedly killed hundreds and devastated the district.

The Homs bloodshed further fuelled calls among Arab countries and the West for action to stop the crisis, which many fear is moving closer to civil war as the opposition turns more to armed resistance.

The UN.estimates that more than 7,500 people have been killed since Syria launched its crackdown on the uprising, which began a year ago as peaceful protests against Assad.

Activists put the toll at more than 8,000.

Source: Al Jazeera and agencies