Syrian activists have reported violence in different parts of the country, saying nearly 40 people were killed.
The deadliest incident was in the northern town of Ariha where a shell hit a home killing seven members of the same family, the UK-based activist network Syrian Observatory for Human Rights (SOHR) said on Sunday.
A video posted online showed the seven men's bodies, some badly mutilated, including one who had part of his head blown off.
Activists also reported intense shelling and clashes between rebels and troops in the eastern city of Deir Az zor and the central city of Homs, which has been under a government attack for the past two weeks.
Earlier on Sunday, activists said rebels captured a military base in the northern province of Aleppo, confiscating large amounts of ammunition.
Deaths near base
The SOHR said 16 government troops died in the attacks on the base near the rebel-held town of Daret Azzeh and nearby checkpoints early Sunday.
Mohammed Saeed, a local activist, said the rebels had removed hundreds of artillery shells from the base. Saeed said via Skype that troops retaliated with intense shelling on the area using helicopter gunships.
State media said 25 people were kidnapped on Friday by "terrorists" and killed in Daret Azzeh. Activists said the 25 killed were pro-regime armed men known as shabiha.
In-depth coverage of escalating violence across Syria
Syria's state-run SANA news agency, meanwhile, said armed men from Turkey clashed with Syrian border guards in Rabiah, a region in the coastal Latakia province.
SANA said several infiltrators died in the late Saturday clash, while others reportedly returned to Turkey.
It said several Syrian border guards were hurt, but didn't specify how many.
Turkey denies sheltering armed Syrian rebels, although many Syrian refugees have fled to camps on the Turkish side of the border.
Also on Sunday, Syrian opposition groups met in Brussels to hash out differences and plan for a democratic transition.
The disparate groups are divided over whether outside military intervention would help or hurt and whether to engage in dialogue with Assad's regime. The conference, attended by some 50 people, will continue on Monday.
In Sunday's other Syria-related developments, Sameeh Maaytah, Jordan's information minister, said that three other Syrian pilots had defected last week, even before a pilot flew his fighter jet into neighbouring Jordan.
He said the other three crossed overland into Jordan.
He was unsure if the four pilots knew each other or had co-ordinated their escape from Syria.
Meanwhile, a ship carrying Russian helicopters to Syria, which turned back after its insurance was cut, was expected to resume its journey accompanied by at least one other vessel, the Interfax news agency reported, citing a military source.
The report is likely to reignite international criticism of Russia's arms deliveries to Syria, which US officials have called reprehensible, and which the Arab League has said should be stopped.