Syrian rebels fighting the regime of Bashar al-Assad have withdrawn all their main fighting units from their stronghold in the Salahedin area of Aleppo, after heavy shelling by government forces.

The pullout from the northern city was ordered just after sunrise on Thursday after a night of intensive attacks by planes and tanks on all three rebel frontlines.

Free Syrian Army (FSA) commanders claimed the withdrawal was tactical and said a small force had stayed behind to oppose any advance by government forces. But the move seemed to mark a significant moment in the fight for control of southern Aleppo, which had raged for more than two weeks, claiming several hundred casualties.

In Damascus, meanwhile, the FSA announced the defection of Muheddine Musalmani, President Assad's protocol chief. But a state TV channel said he would issue a statement denying the claim.

Last week, Assad's prime minister, Riyad Hijab, a veteran Ba'ath party member, became the most senior Syrian political figure yet to defect, appearing in Jordan on Wednesday. Hijab's replacement was named on Thursday as Wael al-Halqi, formerly the health minister.

Iran – a close ally of Syria – called for "serious and inclusive" talks between Assad's government and the opposition as it hosted a conference for 29 countries that it said had "realistic" views on the escalating crisis. None of the participating countries has called on Assad to quit or backed opposition groups. Only Iraq, Pakistan and Zimbabwe sent their foreign ministers.

Fighting was reported on Thursday in the Damascus area, Hama and Deraa. Shelling also intensified in other parts of Aleppo which had notionally been held by the FSA since it secured its foothold in Salahedin in late July.

The regime push had been widely expected earlier this week and rebels had vowed to defend their ground. However, commanders said the intensity of the shelling meant their fighters now had too few opportunities to take cover.

"They were shelling the buildings at the frontline all night," said Major Abu Furad, who led one of the main units in Salahedin, Katibat al-Ansar. "The buildings were falling in front of us. It was impossible to stay there."

The rebel group had been occupying an area known as street 15, which faced a regime force around 200 metres away. Both sides had traded small arms fire for the past fortnight, but neither had advanced beyond their positions. "We withdrew to street 10, which [runs parallel] behind street 15, 150 metres back," he said. "But the regime was shelling that street too. It was relentless and no one could stay behind. They have started using bigger bombs from planes. They were far bigger than the shells from the tanks."

The FSA leadership in Aleppo said it was now redeploying its forces to other key areas of the city. It claimed its ousting from Salahedin had not damaged morale and maintained that the regime was continuing to hold back its ground forces because it fears many would defect if they were ordered to enter the city.

"That's what the defectors are telling us," said Sheikh Tawfik Abu Sleiman, leader of one of the Aleppo brigades. "They would shed their uniforms and run away."

Shelling intensified throughout Thursday in other areas of Aleppo that had previously seen limited fighting. The FSA said it was responding by launching attacks in the centre of the city, where regime troops and the loyalist shabiha militia were stationed to defend key state institutions.

The FSA in Aleppo seemed to be united in a plea for an internationally enforced no-fly zone, which would ground the Syrian air force jets now playing an increasing role in the campaign.

"That's all we need," said Abu Hanefa. "No guns, no armies, just the ability to get these things out of the sky. They are killing us." Another rebel leader, who had left Salahedin, said: "Even the bravest among us had to acknowledge that these jets are something that we cannot fight. It is one thing being outgunned by tanks, but planes are creating a very, very difficult situation."