Clashes between government troops and rebels are raging in opposition bastions of the besieged city of Aleppo as President Bashar al-Assad's key state backer Iran prepared to host a gathering of allies for talks on how to end the conflict.

The regime pressed its new assault on Aleppo, Syria's largest city and commercial hub, for a second day. But blistering attacks on rebel positions from the ground and the air appear to be only slowly chipping away at the opposition's grip on its strongholds.

The state news agency claimed Wednesday that Assad's force had regained control of the Salaheddine neighbourhood, the main rebel area in Aleppo. But activists said rebels were still putting up a fight there on Thursday.

"It's difficult to know exactly what's going on because of the scale of the bombing, but the rebels are still fighting," Aleppo-based activist Mohammad Saeed told The Associated Press by Skype.

He said troops were using warplanes and tanks to shell the towns of Hreitan and Tel Rifat, some 40 kilometres north of Aleppo, from where most of the rebels converged on the city.

"They are trying to cut the main lines from Tel Rifat to Aleppo," he said.

Syrian fighter jets launched airstrikes Wednesday on Tel Rifat, hitting a home and a high school and killing six people from one family, residents said.

They said government forces often shelled the village, but that this had been the first airstrike. They acknowledged that there were some rebels in the village, though an Associated Press reporter saw no armed men during a brief drive through the area.

Aleppo holds great symbolic and strategic importance. Some 40 kilometres from the Turkish border, it has been a pillar of regime support during the uprising. An opposition victory there would allow easier access for weapons and fighters from Turkey, where many rebels are based.

Iran holds conference of 'friends' seeking peace

State television in Iran, Syria's closest ally in the Middle East, said Tehran was hosting a conference of "friends" of Syria in the hope of finding a peaceful solution to the crisis.

Hossein Amir Abdollahian, deputy foreign minister in charge of Arabic and African countries, said representatives from 15 countries will attend, including ones from powerful Russia and China, as well as Pakistan, Iraq, Algeria and Venezuela. Russia's Foreign Ministry said Moscow will attend the talks hosted by Iran, represented by its ambassador in the capital Tehran.

The meeting was called at short notice.

Speaking at the opening of the conference in Tehran, Iranian Foreign Minister Ali Akbar Salehi said his country rejects any foreign and military intervention in Syria and accused rebels of kidnapping nationals of other countries and using civilians as "human shields."

Syrian rebels last week intercepted a bus carrying 48 Iranians in a Damascus suburb and seized them. Rebels claimed the men are military personnel, including some members of Iran's powerful Revolutionary Guard, who were on a "reconnaissance mission" to help Assad's crackdown on the uprising.

Iran initially said the 48 were pilgrims visiting a Shiite shrine in Damascus. Salehi said Wednesday that some of the kidnapped Iranians are retired members of the army and Revolutionary Guard.

The overwhelmingly Sunni Muslim rebels have also abducted 11 Lebanese Shia pilgrims who have been held in northern Syria since May.

Assad, meanwhile, appointed a new prime minister to replace the one who defected to neighbouring Jordan this week in a humiliating blow to the regime. State-run news agency SANA said he appointed Health Minister Wael Nader al-Halqi, a Sunni member of the ruling Baath party who hails from the southern city of Daraa, birthplace of the 17-month-old uprising.

He replaces Riad Hijab, who defected to Jordan this week. Like nearly all prominent defectors so far, Hijab is a member of Syria's majority Sunnis — the Muslim sect which forms the bedrock of the uprising.

Still power remains closely held within Assad's inner circle and the leadership is dominated by members of the president's minority Alawite sect, an offshoot of Shia Islam.