Syrian government forces have launched an all-out offensive on rebel positions in Damascus, as the UN Security Council extended for 30 days the mandate of its observer mission.
Rebels also clashed with troops in several neighborhoods of Syria's second-largest city, Aleppo, on July 20, a day after rebels seized a number of crossings on the Iraq and Turkey borders.
Thousands of civilians meanwhile continued to flee to Lebanon and Iraq to escape the violence.
The UN refugee agency said between 8,500 and 30,000 Syrians had entered Lebanon in the past 48 hours, and thousands of Iraqis have also returned home, after having fled to Syria from their own country's violence.
Activists put the two-day death toll at more than 470 people, one of the highest of the 16-month uprising.
The Security Council meanwhile voted unanimously to give a "final" 30-day extension to its observer mission.
The monitors had been charged with overseeing a ceasefire that never took hold, but suspended their mission on June 16 in the face of escalating violence.
The July 20 vote left the door open for a further but unlikely extension of the mission.
Vitali Churkin, the Russian ambassador to the United Nations, said after the vote, "I was somewhat surprised to hear [U.S.] Ambassador [Susan] Rice's definition, or description, of the resolution as something which provides for an orderly withdrawal of the mission."
Churkin added, "This is not a resolution about withdrawal, it's a resolution about a continuation of the activity of the mission. If a decision were to be taken not to extend the mission, then of course it can withdraw very quickly and I hope safely, but this is not about withdrawal and I think we should not disorient the mission and the international community by describing it as a withdrawal resolution."
The vote followed emergency consultations just hours before the expiry of the 300-strong mission's mandate, after Russia threatened to use its veto powers as a council permanent member for the second time in as many days.
UN Secretary-General Ban Ki-moon said on July 21 that Syrian authorities have "manifestly failed" to protect civilians and called on the international community to act to stop the violence.
Speaking during a visit to Croatia's Brijuni Island as part of a week-long Balkans tour, the UN chief said the Syrian government must stop the killing in the country and its use of heavy weapons against the population.
In Damascus, forces loyal to President Bashar al-Assad recaptured the neighborhood of Medan, while fierce fighting continued in at least three other parts of the capital.
Troops and rebels clashed for a second day in the Syrian city of Aleppo on July 21. The London-based Syrian Observatory for Human Rights says in Aleppo "battles have been ongoing since Friday morning between regular forces and fighters from rebel units in the Salaheddin neighbourhood."
Rights group Amnesty International has said rebel forces too could be blamed for the deaths of civilians as they extended the fighting into large cities.
Assad's national security chief, General Hisham Ikhtiyar, died on July 20 from wounds sustained in a July 18 bombing that killed three other key aides to Assad, including the defense minister and Assad's brother-in-law.
Assad has only appeared publicly in one soundless video on state TV since the bombing, and his whereabouts remain unknown.
The revolt against Assad's regime began in March 2011 and has killed more than 17,000 people.