A ferocious dawn battle on Saturday on the outskirts of Raqa, a strategic Syrian city near Turkey, killed dozens of troops and rebel fighters despite a flurry of diplomacy aimed at halting the bloodshed.
"Fierce clashes pitting rebel fighters from several battalions against regular troops have raged since dawn on the outskirts of Raqa city," said the Syrian Observatory for Human Rights.
"Army troops shelled several city neighbourhoods, as well as the outskirts, while the clashes left dozens of troops and rebels dead," the Britain-based group said without giving exact numbers.
"Explosions could be heard in the city, and towers of smoke could be seen rising into the sky.
Both the Observatory and activists in Raqa said the army was using helicopters to strafe rebels in some parts of the city, in a rare escalation of violence in the provincial capital.
Raqa is located near Syria's northern border with Turkey and Observatory chief Rami Abdel Rahman said it has become home to 800,000 people who were forced to flee other war-torn parts of Syria.
Dawn mortar attacks on Saturday on Raqa's Masaken Shuhada area forced many people to flee, Abdel Rahman told AFP.
"Because of the large number of civilians who have sought shelter in Raqa from other parts of Syria, the rebel Free Syrian Army had an agreement not to assault the city," he said.
"Saturday's battle was a rare but intense escalation."
Fresh battles also rocked Daraya, a key rebel enclave southwest of Damascus which the army has fought to take back from insurgents for several weeks, said the watchdog.
The army also pounded the nearby rebel-held town Moadamiyat al-Sham, as well as Douma northeast of Damascus and Yabrud near the Lebanese border, it said.
The UN says Syria's war has killed more than 70,000 people in nearly two years since the outbreak of an uprising against President Bashar al-Assad's regime.
Despite the spiralling bloodshed and a week of international efforts to reach a compromise, world powers have so far failed to secure an agreement to end the war.
The White House announced Friday that President Barack Obama will visit Saint Petersburg in September to attend a G20 summit and meet Russian counterpart Vladimir Putin at the G8 summit in Northern Ireland in June -- after the two leaders spoke by phone to discuss the Syria crisis.
Washington and Moscow have been especially at odds over Syria, and Russia's role in vetoing UN Security Council action to censure Assad.
"The two presidents agreed on the need to advance a political transition to end the violence (in Syria) as soon as possible," said a White House statement.
US Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov will continue to work together on the issue following their meeting in Berlin on Tuesday, it added.
A Kremlin statement said, meanwhile, that Putin noted the need to end "military activities" in Syria as soon as possible.
The latest developments come after the US on Thursday pledged to provide direct aid to the rebels in the form of food and medical assistance, as well as $60 million in funding for the political opposition, but no arms.
That decision triggered criticism from Russia, the official Syrian media and even opposition leaders.
Russia's foreign ministry said the decision would backfire and encourage "extremists to take power by force."
But senior opposition figure Burhan Ghalioun told AFP it was the very failure to arm the rebels that "favours the rise of extremists," saying he hoped Washington would realise this was a "mistaken policy."
Syrian official media also poured scorn on the US announcement, with Ath-Thawra daily saying on Saturday it was part of Washington's "policy of lies and deception" on Syria.
On Friday alone, violence across Syria killed at least 142 people, among them 46 civilians, the Observatory said.