Unrest in Syria cost at least 15 lives on Friday and troops backed by tanks attacked Zabadani, a town near the border with Lebanon, an opposition leader said, in the first big military assault since Arab League monitors arrived last month.
The League chief said he feared a 10-month-old struggle to topple President Bashar al-Assad could slide into civil war.
The Arab League mission to Syria has been struggling in recent days, with some of its observers starting leave in protest at continuing violence directed against anti-government protesters by security forces loyal to Assad.
Tanks are bombarding the town and have entered the outskirts, but they are being met with resistance, Kamal al-Labwani, an opposition leader from Zabadani who fled to Jordan two weeks ago, told Reuters. The Free Syrian Army (army defectors) has a strong presence in the area.
The Syrian Observatory for Human Rights said seven civilians were shot dead, four of them in the restive city of Homs. Three others died, including two wounded earlier in the week.
A total of five security personnel, including a lieutenant colonel, were killed and 15 wounded by armed terrorist groups in two separate attacks in Homs and the countryside around Damascus, the state news agency SANA reported.
The people want the downfall of the regime, shouted a crowd in the port of Latakia at one of several anti-Assad demonstrations that erupted after Friday Muslim prayers.
Syrians have kept up a campaign to end four decades of Assad family rule since March despite a crackdown by the authorities that the United Nations says has killed more than 5,000 people.
Some, including army deserters, have taken up arms in recent months. Syrian authorities say foreign-backed terrorists have killed 2,000 soldiers and police since the revolt began.
Armed clashes, now punctuating what began as a non-violent protest movement, have raised fears of a full-scale conflict in Syria, a Sunni Muslim-majority country of 23 million which also has Alawite, Druze, Christian and Kurdish minorities.
Yes, I fear a civil war and the events that we see and hear about now could lead to a civil war, said Nabil Elaraby, head of the Arab League, which deployed monitors on December 26 to check whether Syria was respecting an Arab peace plan.
Any problems in Syria will have consequences for the neighbouring states, he told Egypt's Al-Hayat television.
Syria, which borders Lebanon, Turkey, Iraq, Jordan and Israel, is at the heart of the conflict-prone Middle East, where its closest allies are Iran and the Lebanese Hezbollah group.
The Syrian authorities must respond to the legitimate democratic aspirations of the Syrian people, U.N. Secretary-General Ban Ki-moon, who is visiting Lebanon, was quoted as saying by the Beirut newspaper an-Nahar.
He also urged the Security Council, where Russia and China have blocked firm action on Syria, to speak with one voice.
The most senior Syrian officer to defect to the opposition told Reuters that desertions were wearing down the army, but that rebels could take more than a year to topple Assad.
General Mostafa Ahmad al-Sheikh said that up to 20,000 soldiers, mostly Sunnis, had left despite iron controls, although most were more focused on evading capture by the secret police than on fighting the security forces.
He said the revolt would take longer than those that toppled leaders in Libya, Egypt and Tunisia because Assad retains the loyalty of elite forces from his minority Alawite sect.
Video footage posted on the Internet on Friday showed the burning hulk of an armoured personnel carrier in Homs, a hotbed of protests and armed resistance to Assad. A voice on the clip said the Free Syrian Army (FSA) mounted the attack.
The main opposition Syrian National Council (SNC) said it had agreed with the FSA to open a direct channel of communication with the rebel army, whose loosely-structured units would also be reorganised.
SNC leader Burhan Ghalioun has said rebel attacks could push Syria towards civil war, but his comments have made little impact on rebel fighters who operate largely independently of their nominal leadership, based across the border in Turkey.
Syrian opposition groups, and at least one disgruntled monitor, say the Arab League monitoring mission has only bought Assad more time. Arab foreign ministers are to due to hear a report from the monitors on January 19 and decide what to do next.
Elaraby said the bloodshed had abated somewhat since the observers arrived. That contradicts the view of a senior U.N. official said to have told the Security Council this week that the rate of killings had accelerated to about 40 a day.
A Russian-operated ship carrying ammunition docked in Syria this week, after it had been temporarily halted during a refuelling stop in Cyprus, a Cypriot official said.
France called for an independent investigation into the death of a French TV journalist killed in a mortar attack in Homs this week while reporting on unrest there.
(Additional reporting by Dominic Evans in Beirut, Lin Noueihed in Cairo and Michele Kambas in Nicosia; Writing by Alistair Lyon; Editing by Giles Elgood.)