Military sources in Syria say there are 20,000 government troops massed in and around the northern city of Aleppo, poised for an all-out attack on the rebels.
The government also claims to have re-taken the capital Damascus and the rebels said they had withdrawn.
Fighter jets, helicopter gunships and artillery are already bombarding rebel positions in Aleppo, where they're reported to be facing strong opposition.
The BBC reports the rebels, who have also increased their numbers, are well dug in and continue to try to extend the territory under their control.
Abdel Jabar Oqaida, a commander of the Free Syrian Army (FSA) in Aleppo, told the AFP news agency that the restive Salah al-Din district had "come under the heaviest bombardment since the battle began" on 20 July.
A senior government security official told the agency: "The battle for Aleppo has not yet begun, and what is happening now is just the appetiser... the main course will come later."
The fight for the key strategic city has been intensifying over the last few days, with Syrian state television reporting that troops had inflicted huge losses on what it called "terrorist mercenaries" in Salah al-Din and in other nearby areas.
UN vote criticises Security Council
The violence follows a United Nations General Assembly vote criticising the Security Council for failing to act on Syria.
The UN also condemned the Syrian government for its use of violence.
A non-binding resolution was passed on Friday by 133 votes to 12 with 31 abstentions.
Saudi Arabia was the driving force behind the resolution after the resignation of UN-Arab League envoy Kofi Annan and the failure of his six-point peace plan.
Secretary General Ban Ki-moon said the conflict in Syria had become a "proxy war".
Russia said the resolution undermined peace efforts in Syria. Envoy Vitaly Churkin said the resolution was one-sided and gave blatant support to the opposition.
Russia voted "no" on Friday along with China, Syria, Cuba, Iran, North Korea, Belarus, Bolivia, Nicaragua, Burma, Zimbabwe and Venezuela.
Russia and China have blocked three attempts in the Security Council to impose sanctions against Damascus.