Syrian troops and rebels battled for control of a corridor near the border with Turkey on Saturday, monitors said, as rebel-held areas of Aleppo came under regime artillery fire.
Loud explosions from shelling were heard across the northern city, an AFP correspondent reported.
At least 11 soldiers and five rebels were killed in clashes in the Orm and Kaf Jum areas of the province of Aleppo, near the Turkish border, said the Syrian Observatory for Human Rights.
"The state has no presence except for military and administrative posts" in the western region of the province in northern Syria, Observatory director Rami Abdel Rahman told AFP by telephone.
He said the regime was determined to prevent rebels linking up between western Aleppo and the neighbouring province of Idlib as this would form an extensive insurgent-held region on the border with Turkey, which supports the revolt in Syria.
In shelling of several rebel-held district of Aleppo city, five members of the same family, including children, were killed in Maysar, said Abdel Rahman, whose Britain-based monitoring group relies on a network of activists on the ground.
The Observatory said regime artillery gunners targeted the districts of Katergi, Shaar, Sakhur, Hanano, Arkub and Marjeh.
In the central city of Homs, where rebels remain entrenched in many areas, at least one soldier was killed in clashes near the Old City, while a civilian also died in Juret al-Shiyah district, the Observatory said.
Outside the city, four soldiers were killed in an attack on their vehicle and a man was killed elsewhere in the province, it said, adding that three rebels were killed in clashes in the flashpoint town of Rastan.
Elsewhere in the country, security forces carried out arrests and raids in the town of Hara in the southern province of Daraa while fighting broke out in the village of Dael.
National Coordination Committee for Democratic Change head Hassan Abdel Azim, meanwhile, told AFP that three members of the normally tolerated opposition group who went missing Thursday on the Damascus airport road were probably detained by Syrian intelligence.
In Beirut, the Lebanese military said "a large number" of Syrian rebels attacked one of its posts near the border with the war-torn country on Friday night, without causing any casualties.
"Army reinforcements were dispatched to the area and began to pursue the gunmen, who fled after the attack towards the mountains and several border towns and villages" inside Lebanon, it said in a statement.
The Syrian Observatory gave an initial toll of at least 31 people killed nationwide on Saturday: 10 civilians, 12 soldiers and nine rebels. It reported a total of 142 people, including 88 civilians, were killed in violence on Friday.
The opposition Syrian National Council warned on Friday that the deadly conflict which broke out in March 2011 was reaching "a point of extreme gravity" that could trigger higher levels of extremism in neighbouring states.
The conflict could lead to "a catastrophic situation, with more extremism and damage also in neighbouring countries," SNC head Abdel Basset Sayda told reporters in Rome.
According to the Observatory, at least 29,000 people have been killed since the revolt against President Bashar al-Assad's rule erupted.
But as the violence raged unabated, a top NATO general said in Brussels that the alliance does not believe that military intervention would bring any improvement in Syria's security situation.
Germany's Manfred Lange, Chief of Staff of Supreme Headquarters Allied Powers Europe, said the "political process has to be pushed forward, sanctions need to take effect. At the moment, this situation cannot be solved by the military in a responsible way."
France is still discussing with allies whether to try to set up a no-fly zone in Syria to help rebels under assault from regime forces, a top French official said.
"We are working -- but not only us, a lot of countries are working -- on the issue of a no-fly zone, but for the moment it is clear that it's very difficult to set up for several reasons," said the official, who asked not to be named, during a visit to Washington.