* Fugitive Alawite ex-defence minister reaches Istanbul -sources
* Syrian state media denial not repeated, dismissed by diplomat
* Opposition figure says Habib could play role after Assad (Repeats UPDATE 1 to additional subscribers)
By Amena Bakr and Khaled Yacoub Oweis
DOHA/AMMAN, Sept 5 (Reuters) - Bashar al-Assad's former defence minister has reached Istanbul after a defection that betrays cracks in the president's support among his own Alawite sect, opposition and diplomatic sources said on Thursday.
Dismissing a cursory Syrian state television denial of the first Reuters report that General Ali Habib had been spirited across the Turkish frontier this week, opposition figures said Habib was likely to keep a low profile after evading house arrest and reaching Turkey with the aid of Western agents.
One prominent opposition figure also spoke of speculation that Habib, who is in his 70s and apparently broke with Assad after a crackdown on protesters in 2011, might be lined up by U.S. and Russian officials for a role in transitional arrangements to negotiate an end to the civil war.
"My information, based on a trusted Western source, is that he is in Istanbul," veteran dissident Kamal al-Labwani, now based in Paris, told Reuters on Thursday. "Habib exited with Western intelligence involvement, so do not expect public statements by security operatives on his whereabouts."
A source in one of the Gulf Arab states that is backing the revolt against Assad said Habib had crossed Turkey's southern border after dark on Tuesday and had reached Istanbul, a base for the Western-backed Syrian National Coalition.
A Western diplomatic source said he had confirmation that Habib had defected and was in Turkey. He dismissed a brief report on Syrian state television on Wednesday which said he was still at his home. The station has not repeated that denial and other state media have not mentioned Habib.
The Turkish government, which hosts Syrian military officers who have defected, has not confirmed the general's presence. He has not been seen in public this week.
Alawites, who practice an offshoot of Shi'ite Islam, make up about an eighth of Syria's 22 million population. Many rose to prominence after Assad's father Hafez al-Assad seized power in a coup in 1970. Habib, born in 1939, was once army chief of staff and served as defence minister in 2009-11.
Some opposition sources say that Habib disagreed with the use of force against protesters at the start of the revolt, which began in March 2011, and so he was dismissed as minister that August. Apparently unwilling to publicise the rift, Assad kept Habib quietly under guard while having him appear in public at times to make a show of loyalty, those sources said.
Habib himself was quoted as saying that he had stepped down from the defence ministry on health grounds.
Numerous senior military and political figures have deserted Assad since 2011, but most have been from the country's Sunni Muslim majority, while members of Assad's minority Alawite sect have largely remained loyal to his government.
Western countries have been hoping for a senior Alawite to distance himself from Assad, so that someone could potentially represent the community in a political deal to end the war.
Andrew Tabler, a senior fellow at the Washington Institute, said the defection of such a senior Alawite figure was "a major development" which could help the West lure other members of the community to break with Assad: "There are opportunities here."
But other experts were more sceptical. Peter Harling, a Syria analyst with the International Crisis Group, said that because Habib had been out of office for two years, "it is inaccurate to describe him as someone whose defection would tell us much about the state of the regime today."
Labwani, the veteran dissident, said he believed that both Washington, which backs the opposition, and Moscow, which backs Assad, could act jointly to promote Habib for a post-war role.
"It seems that the Americans - and to a degree the Russians - are preparing him for a post-Assad role," he said, describing one possibility as Habib taking control of government forces and then negotiating with the rebels on a transitional government.
"I am all for this roadmap," Labwani said. "Habib is the right man."
Russia and the United States announced in May they would try to bring Syrian government and opposition representatives together at an international conference, but no date has been set and there is no sign it could be held in the near future.
Accusing Assad of using poison gas last month on rebel-held suburbs of Damascus, U.S. President Barack Obama is seeking congressional approval for limited military action that the White House says is not aimed at overthrowing the government.
Russian President Vladimir Putin, whose country has long armed Assad, has said rebels might have released the toxins and warned Obama against military action without U.N. backing. (Additional reporting by William Maclean and Oliver Holmes in Beirut; Editing by Alastair Macdonald and Peter Graff)