Syrian troops stormed the rebel stronghold of Idlib on Saturday after shelling the city for several days, confirming fears of an assault after another rebel redoubt was overrun last week, monitors said.
The attack came as peace envoy Kofi Annan was in Damascus seeking an end to a year-long crackdown on dissent that has cost an estimated more than 8,500 lives, holding "positive" talks with President Bashar al-Assad.
"Troop carriers entered the city of Idlib as clashes raged" between the army and rebels, the head of the Syrian Observatory for Human Rights monitoring group, Rami Abdel Rahman, told AFP in Beirut.
The army heavily shelled Idlib before sweeping into the city, where 15 civilians were killed, Abdel Rahman said, adding that dozens were wounded and 150 arrested by regime forces.
Earlier, he said 16 rebels were killed in an army ambush as they headed for Idlib and that four regime soldiers died in a separate incident in the province.
The Observatory said 62 persons -- mostly soldiers and rebels -- were killed across Syria, 55 of them in Idlib province.
Local activist Milad Fadl said Saturday's "bombardment began at 5 am (0300 GMT). The shelling is very, very heavy."
Abdel Rahman said "it's the heaviest bombardment since troop reinforcements were sent to Idlib earlier this week."
Regime forces have been massing around the city for days to snuff out rebel Free Syrian Army fighters who are entrenched in the province of the same name.
On Friday, armoured units surrounded the hilly district of Jabal al-Zawiya in the province, where according to Abdel Rahman "the largest number of deserters" are to be found.
Activists expect Idlib to suffer the same fate as the Homs neighbourhood of Baba Amr, which was stormed by government troops on March 1 after a month of shelling and in which an estimated 700 people died.
Just hours before the assault, President Bashar al-Assad promised Annan that he would back any "honest" peace bid but warned dialogue would fail if "terrorist groups" remained.
Syrian state television said there was a "positive atmosphere" to the meeting between Assad and the former UN chief, on his first visit since being named United Nations-Arab League envoy on the conflict.
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"Syria is ready to bring success to any honest bid to find a solution," the official SANA news agency quoted Assad as telling Annan.
But "no dialogue or political process can succeed as long as there are terrorist groups that are working to sow chaos and destabilise the country by attacking civilians and soldiers," he added.
A UN statement said Annan had expressed "grave concern" to Assad over the crackdown and "urged the president to take concrete steps to end the current crisis."
The former UN secretary general "put several proposals on the table regarding stopping the violence and the killing, access for humanitarian agencies and the ICRC, release of detainees and the start of an inclusive political dialogue," said a UN statement.
It gave no details, but said Annan, who will meet Assad again on Sunday, had described his first talks as "candid and comprehensive."
Annan has the support of Damascus allies Beijing and Moscow and his mission has been welcomed by both the Syrian government and the opposition.
But Russia said its Foreign Minister Sergei Lavrov made clear to him at a meeting in Cairo that Moscow was opposed to "crude interference" in Syria's affairs.
And in Cairo Russian and Arab foreign ministers called for an end to the violence "whatever its source" as they struggled to find common ground to end the conflict in Syria.
Reading out a joint statement, Lavrov and Qatari Foreign Minister Sheikh Hamad bin Jassim Al-Thani said they also agreed there should be no foreign intervention in Syria.
The Arab ministers later issued a separate statement condemning the Syrian regime's month-long shelling of the Baba Amr district of Homs as a "crime that reaches the level of crimes against humanity."
The Saudi and Qatari foreign ministers slammed Russia and China's support for Syria at the onset of the Cairo talks.
Saudi Arabia's Saud al-Faisal accused Moscow of giving Damascus a "licence to extend its brutal practices against the Syrian people, without compassion or mercy."
Meanwhile opposition figures who met Annan on Saturday in Damascus insisted that the regime must pull troops from cities and towns and release political prisoners before the start of any dialogue with the regime.
"We cannot talk about a political process before a ceasefire... the release of political prisoners and the withdrawal of troops are withdrawn from cities and towns," one of them, Abdel Aziz al-Kheir, told AFP.