The Syrian government warplanes pounded rebel-held districts of Salaheddin and Sakhur in the northern city of Aleppo on Sunday, a watchdog said.
"Fighter jets are being used to pound Shaar and Sakhur," Rami Abdel Rahman of the Syrian Observatory for Human Rights told AFP.
Earlier, clashes broke out between troops and rebels in the districts of Ansari, Hamdaniyeh and Sukari in Aleppo, scene of heavy fighting since July 20, the Observatory said, adding at least 40 people were killed across Syria on Sunday.
Of those killed, 24 were civilians, 10 were soldiers and six were rebel fighters.
Iran appealed for help from governments with ties to the Syrian opposition in securing the release of 48 of its nationals seized from a bus in Damascus as an Arabic news channel aired footage it said was of the Iranians in the hands of rebel captors who charged that their hostages were Revolutionary Guards.
A high-level security source said the army on Sunday completed its deployment of reinforcements to Aleppo, ready for a decisive showdown.
"The war is likely to be long, because there will have to be street battles in order to get rid of the terrorists," the source told AFP, declining to be named.
"All the reinforcements have arrived and they are surrounding the city," he said. "The army is ready to launch its offensive, but is awaiting orders."
Troops shelled rebel-held Salaheddin district in the southwest and clashes erupted in the Sukkari, Hamdaniyeh and Ansari neighbourhoods, the Britain-based watchdog said.
The opposition Syrian National Council charged that the army's bombardment of the rebels was hitting key public institutions in the commercial capital, some of historical significance.
"After failing to subdue (rebel forces) in Aleppo... the Syrian regime's gangs have started to target government institutions and buildings," the exiled opposition group said. "Some of them have historical and archaeological value."
Aleppo preserves a raft of historical sites, including its renowned 13th century citadel. The UN Educational, Scientific and Cultural Organisation named the Ancient City a World Heritage Site in 1986.
The SNC accused the army of shelling Aleppo's television building.
"The criminal regime does not hesitate to shell these institutions," the group said, adding: "The rebels were forced to move away from the television building in order to protect the Syrian people's property and heritage."
Rebels tried to storm the state television building on Saturday before being driven back by shelling, the Observatory said.
'Just the appetizer'
A senior government security figure warned on Saturday that "the battle for Aleppo has not yet begun, and what is happening now is just the appetizer... The main course will come later."
The official said at least 20,000 troops were now on the ground. "The other side are also sending reinforcements," the official added of the rebels, who claim to have seized half the city.
The pro-government Al-Watan newspaper said on Sunday that the army had killed "hundreds of terrorists" in Aleppo but that between 6,000 and 8,000 remained in the city.
On Saturday, violence nationwide killed 205 people -- 115 civilians, 38 rebel fighters and 52 soldiers, the Observatory said.
Because of restrictions on the free movement of journalists in Syria, it is impossible to independently verify death tolls and claims of either side.
The Syrian army said on Saturday it had seized the last rebel-held district of Damascus and authorities took journalists on an escorted tour.
A brigadier general told reporters the Tadamun neighbourhood had been retaken after heavy fighting and that the military now controls all of the capital.
"We have cleansed all the districts of Damascus, from Al-Midan to Mazzeh, from Al-Hajar Al-Aswad to Qadam... to Tadamun," said the officer.
Iran appealed to Qatar and Turkey, which both have close relations with the Syrian opposition, for help in securing the release of 48 nationals it says were seized while on pilgrimage to the Sayyida Zeinab shrine in Damascus.
Al-Arabiya television aired footage on Sunday which it said was of the Iranians in the captivity of rebel Free Syrian Army fighters who charged that their hostages were elite Revolutionary Guards.
Fighters of the Al-Baraa Brigade of the rebel Free Syrian Army have "captured 48 of the shabiha (militiamen) of Iran who were on a reconnaissance mission in Damascus," said a man dressed as an FSA officer in the video screened by the Dubai-based channel.
"During the investigation, we found that some of them were officers in the Revolutionary Guards," he said, showing documents taken from one of the men, who appeared in the background.
Tehran, Damascus's key regional ally, has repeatedly denied it has sent any military units to Syria.
In the latest reported high-level defection, General Muhammed Ahmed Faris, a military aviator who became the first Syrian in space, fled to Turkey on Sunday, the Turkish news agency Anatolia reported.
On the diplomatic front, US Secretary of State Hillary Clinton is to travel to Turkey next Saturday for talks on the conflict in Syria, a State Department spokeswoman said.