Fighter jets have bombed eastern areas of Syria's second city Aleppo, a BBC reporter near the city says.
The attack, which followed an artillery barrage, is seen as a significant escalation in the conflict.
Rebels launched an offensive against Aleppo at the weekend in an attempt to wrest the city from government forces.
US Secretary of State Hillary Clinton says territorial gains made by the rebels will eventually result in a "safe haven" inside Syria.
"We have to work closely with the opposition, because more and more territory is being taken," she told reporters in Washington. That would become a safe haven which would provide a base for further action by the opposition, she said.
Mrs Clinton said the pace of events was accelerating inside Syria and called on the opposition to be prepared for a transition of power, adding that it was not yet too late for President Assad himself to prepare for such a handover.
In one of the deadliest incidents of the day, opposition activists reported that at least 20 worshippers were killed as they went into a mosque in a village close to the city of Hama.
Troops and militia loyal to President Assad left a roadblock and opened fire on the men who were arriving for evening prayers in Shariaa, one activist told Reuters news agency.
Foreign journalists work under intense restrictions in Syria so reports by both sides are hard to verify.
Earlier in Aleppo, government forces launched what correspondents described as a co-ordinated attack on the Tariq al-Bab area in the east of the city late on Tuesday afternoon, first with a 10-minute barrage of 30 shells and then with bombing runs from fixed-wing jets.
It was thought to be the first time that Syrian fighter planes had been used for bombing urban targets during the conflict, as the government attempted to take back districts of Syria's commercial centre seized by the rebels.
There are reports of dozens of casualties and widespread damage. The BBC's Ian Pannell, on the outskirts of Aleppo, says civilians and fighters are among the dead and wounded.
Helicopter gunships have been involved in the clashes in the city throughout the day, he says.
Fierce fighting has been reported close to the historic Old City. A French correspondent there has spoken of rebels besieging a police headquarters close to the walls of the Old City, which is a world heritage site.
Government forces have already regained control of most areas of Damascus that were captured by rebels last week. Opposition activists report renewed raids by troops in the Tadamon, Qadam and Assali areas of the city.
State television has broadcast video of soldiers apparently securing and checking heavily shelled suburbs in the south of the capital.
The battle for Aleppo, Syria's most populous city, appears to have spread to much of the city, with rebels claiming control of several areas.
State forces appear to have focused their counter-attack on the Sakhour area to the east of the city centre, where rebels destroyed several tanks on Monday, the BBC's Jim Muir reports from neighbouring Lebanon.
The violence on Tuesday claimed the lives of more than 100 people in Syria, according to the opposition Local Co-ordination Committees, including 20 in Aleppo.
Thirteen people died in a revolt in Aleppo prison, activists said, when security forces reportedly opened fire and used tear gas on detainees.
Explosions and fires have also been reported from the jail in Homs where a similar mutiny took place.
Unverified reports said hundreds of refugees had become caught up in violence while trying to flee the country across its eastern border with Iraq.
Three people were wounded when Syrian forces shelled the Syrian side of the Boukamal crossing on the Euphrates river, where a crowd of Iraqis and Syrians were waiting to leave, a source has told the BBC.
The deteriorating security situation has prompted 10,000 Iraqi refugees to leave Syria in less than a week, according to the UN refugee agency.
Elsewhere, 10 people were reported killed when a shell hit their car near Hama and a family was said to have died during a bombardment of Deraa in southern Syria.
As the battle for control of Syria's biggest cities spreads, international concern about Syria's chemical weapons stockpile has spread to its ally, Russia.
The US and the UN have already issued a warning to Damascus, after a Syrian spokesman said its weapons would never be used internally but could be deployed against "external aggression".
Russia's foreign ministry said on Tuesday that it "proceeds from the assumption that the Syrian authorities will continue to rigorously abide by their international obligations".
In other developments
- President Assad named internal security head Gen Ali Mamlouk as his new head of national security, replacing Hisham Ikhtiar, one of four senior officials killed in the 18 July bombing in Damascus
- A commander of Iran's Revolutionary Guards, Brig Gen Masoud Jazayeri, said "decisive blows" would be struck against Syria's enemies, "particularly the hated Arabs", if they intervened in the conflict, Iran's semi-official Fars news agency reported