Fighting raged on Friday in Syria's key cities, a day after the United Nations formally ended its observer mission in the war-torn country, according to opposition sources, dpa reported.
Sounds of explosions rocked the capital Damascus and were followed by clashes near a military airport between President Bashar al-Assad's forces and rebels fighting for his ouster, said activist Haytaham al-Abdallah.
Fierce fighting also continued in Aleppo, Syria's biggest city, for a third consecutive week.
"The intensity of the shelling by the regime in Aleppo has increased in the past 24 hours," Abu Omar al-Halabi, a commander in the rebel Free Syrian Army commander, said.
"The regime troops are using high-calibre shells that can bring down a building of six floors in one shell," he told dpa by phone from Aleppo.
The battle for Aleppo, Syria's commercial hub, could decide the course of the 18-month conflict.
Elsewhere, at least seven people were killed by government forces in the town of al-Quseir, in the dissident province of Homs in central Syria, reported activists.
Around 250 people were killed across Syria on Thursday, reported the Syrian Observatory for Human Rights, a London-based opposition group.
The surge in violence came just hours after the UN Security Council decided not to extend the mandate of its observer mission in Syria.
The observers were dispatched to Syria in April to monitor a ceasefire, which never held.
Lakhdar Brahimi, an Algerian former foreign minister, has agreed in principle to become the new UN special envoy for Syria, according to Western diplomats at UN headquarters in New York.
Brahimi, 78, would take over on September 1 from Kofi Annan as Syria crisis envoy for the UN and the Arab League.
The Western diplomats added that Brahimi was still discussing the details of his proposed mandate.
In Lebanon, meanwhile, the powerful Shiite al-Mokdad clan released 18 of the 40 Syrians it had kidnapped on Wednesday in a bid to secure the release of one of its relatives held by rebels in Syria.
"At this moment, we have halted all operations on Lebanese territory ... because we have a sufficient number of Syrians linked to the (rebel) Free Syrian Army," said Maher al-Mokdad, a member of the clan.
The kidnappings have raised concerns that the Syrian conflict is spilling over the border into Lebanon, which, like Syria, is divided along sectarian lines.