France has said that Russia and China must act within the United Nations Security Council to stop a possible bloodbath in the Syrian city of Aleppo.
Troops loyal to the Syrian president are stepping up efforts to crush rebels threatening the government's two main power centres of Damascus and Aleppo.
Fixed-wing combat aircraft were seen in action over Aleppo, the commercial capital which has a population of 2.5 million.
Residents in the capital reported a shell landing in southern districts every minute this morning.
French Foreign Minister Laurent Fabius said that the world, in particular the permanent members of the UN Security Council, should come together and take their responsibilities.
He singled out Russia and China, saying: "We hope that finally they will hear the cries, not only from the people in Syria, but from the rest of the world and the Arab countries, to stop this bloodbath."
Fierce clashes raged in the early hours in Aleppo, and an activist said rebels now controlled half of the city.
Activists said 24 people were killed in fighting in and around Aleppo yesterday, swelling a national death toll of about 18,000 since the revolt against President Bashar al-Assad began 16 months ago.
Reports from Syria are hard to verify because of restrictions placed on foreign media.
Meanwhile, one of the most senior figures to defect from Mr Assad's inner circle, Brigadier General Manaf Tlas, has put himself forward as someone who could help unite the fragmented opposition inside and outside Syria on a blueprint for a transfer of power.
Syria's ambassadors to the United Arab Emirates and Cyprus were the latest to defect to Qatar, according to spokesmen for the opposition Syrian National Council.
The two, who are husband and wife, are the second and third senior diplomats to defect since the uprising began.
A military attaché at the Syrian embassy in Oman told Al Jazeera television he had defected also.
"After the killing and bombardment and destruction of our people in Syria increased, and the massacres took place, this ... pushed me to defect from this regime," Mohammad Tahseen al-Faqir said.