A Syrian general who was a leading member of Bashar al-Assad's inner circle, Manaf Tlass, has defected to Turkey, according to a Damascus-based website with close links to the regime.

Tlass is a member of the most powerful Sunni family in Syria, and the son of a long-serving former defence minister, Mustafa Tlass, but he was reported to have fallen out of favour in recent months for refusing to take part in attacks on civilian areas regarded as opposition strongholds.

Tlass's defection was reported by Syriasteps, a news website linked to the country's security apparatus. It said that "a highly placed source in intelligence has confirmed that General Manaf Mustafa Tlass has fled to Turkey", and quoted a security official as saying: "His escape does not mean anything."

The defection of such a high-profile figure from a family at the heart of the regime would be a damaging blow to Assad and could provoke more defections, especially among more junior Sunni officers and rank-and-file soldiers. That would serve to weaken the security apparatus, but at the same time sharpen the sectarian nature of the conflict between the Sunni majority and the Alawite minority, from which the ruling family and the military elite are drawn.

A Turkish government official confirmed that two Syrian generals had defected in the past three days, but did not provide names "for their and their families' security".

One of the two generals the official referred to is from an engineering division. The second is believed to be Tlass, who is a general in the Republican Guard.

Rami Abdul-Rahman, the head of the London-based Syrian Observatory for Human Rights, said: "Several sources inside Syria, including Alawite sources close to the regime, have confirmed to me that Manaf Tlass has left the country."

Joshua Landis, a Syria expert at the University of Oklahoma, wrote on his Syria Comment blog that Tlass's "Honda had supposedly turned up in the Rukn ad-Din neighbourhood of Damascus, but he was nowhere to be found".

"Manaf Tlass's father and brother, Firas, who is a leading businessman, are believed to be out of the country. The father had gone to Paris on the pretext of getting medical attention. Firas is said to be in Dubai. It is not clear where the women and children of his family are," Landis wrote.

The steady stream of defections are just one sign of the Assad regime's gradually eroding power. A senior British diplomat said it was also losing its grip on territory.

"What is clear is that the regime has lost control of parts of the country, particularly in the east. It's also clear that parts of Damascus have got more difficult for the regime, but we don't have a precise map … but I'm not sure it's true that the opposition are in full control themselves in a coherent way," the diplomat said.

Iraq's foreign minister, Hoshyar Zebari, said on Thursday he had "solid information and intelligence that members of al-Qaida's terrorist network" have gone to Syria". He added: "Our main concern, to be honest with you, is about the spillover about extremist, terrorist groups taking root in neighbouring countries."

Rebel fighters in Idlib province denied that foreign fighters were a significant factor in the conflict.

"I swear to God that I have not seen one strange Arab here and I've been fighting for more than a year," said Anis Azir, a Free Syrian Army leader in Qurqaniya village. A second guerilla leader, Abu Mahmoud from nearby Athma village, said: "They would not be welcome even if they did come. We know what they represent."

A third rebel leader in Darat Azzah, near Aleppo, was more circumspect. Abu Ahmed, as he preferred to call himself, paused for almost a minute before answering whether al-Qaida would be accepted in his town. "They haven't tried. But we would welcome their weapons."

The senior British diplomat described the role of foreign jihadists as minor but warned that their influence would grow if the international community did not take concerted action against the regime. Russia and China have so far blocked any punitive measures being imposed by the UN security council.

"Our assessment is that the vast majority of people fighting on the opposition side are still Syrians and most are trying to defend their neighbourhoods under intense military pressure," the diplomat said. "But it's clear that there are some other elements getting into this conflict and what we have consistently said to Russia, China and others is that what they say they want to avoid – a descent into an increasingly sectarian conflict with other players, a breakdown of Syrian society with regional ramifications – the chance of all of that happening increases with every day the conflict continues in its current form."

Hillary Clinton, William Hague and foreign ministers from other western and Arab states are taking part in a "friends of Syria" meeting on Friday to discuss further ways of exerting pressure on Damascus. Gulf states will be asked to impose more sanctions, and western capitals may table a new security council resolution calling for a UN-sponsored peace plan to be backed by global sanctions if Syria does not comply. Western diplomats concede Moscow is likely to veto such a move.