BEIRUT/AMMAN (Reuters) - A Syrian military source accused Turkey on Monday of helping Islamist rebels to stage an assault on Idlib, a provincial capital which fighters seized at the weekend.

The source declined to comment on the situation in Idlib, citing security considerations, but a monitoring group has confirmed the al Qaeda-linked Nusra Front and allies now control Idlib and said the Syrian air force bombed the city on Monday.

The fall of Idlib, 30 km (20 miles) from the Turkish border, marks only the second time in the Syrian civil war that Damascus has lost control of a provincial capital. The first was Raqqa, which the ultra-radical Islamic State group has turned into the de facto capital of its self-declared caliphate.

Echoing more general comments by President Bashar al-Assad, the military source accused both Turkey and Jordan of supporting the insurgents in their Idlib offensive, saying they were "leading operations and planning them". The insurgents were using advanced communication apparatus that had been supplied to them via Turkey, the source added.

The Turkish foreign ministry declined to comment.

Turkey is one of the regional states most hostile to Assad, along with Saudi Arabia and Qatar. In an interview with the U.S. network CBS, Assad described Turkish President Tayyip Erdogan as a "Muslim Brotherhood fanatic" who was directly supporting insurgents "logistically and militarily" daily.

Syria's government has not commented on the fall of Idlib, though the pro-government Watan newspaper published in Damascus reported on Monday that the Nusra Front and its allies had raised the al Qaeda flag over government buildings in the city.

Nusra Front was joined by groups including the hardline Ahrar al-Sham in seizing Idlib on Saturday, a reminder of the pressures facing Damascus after more than four years of crisis.

The Islamist alliance rejected a call by the Turkish-based mainstream political opposition to let an interim government set up its headquarters in Idlib, saying it would ruled by those "in the trenches and not in hotels".

Story continues

The Syrian Observatory for Human Rights, the monitoring group, said warplanes staged a number of raids on Idlib. More than 170 people on both sides were killed in the fighting, including at least 126 Syrian insurgents, the Observatory said.

GOVERNANCE QUESTION

The setback for Assad in Idlib coincides with growing tension over Yemen between Saudi Arabia and Iran, one of Assad's firmest backers. Turkey has come out in support of a Saudi-led military intervention against Iranian-allied Houthis in Yemen.

The Syrian conflict, in which an estimated 220,000 people have been killed, has been a major arena for Iranian-Saudi rivalry.

The Nusra Front's influence in northwestern Syria has been expanding at the expense of mainstream rebel groups, some of which received U.S. military support. It is a rival of Islamic State, an al Qaeda offshoot that has seized territory in both Syria and Iraq.

Ahrar al-Sham issued a statement urging the people of Idlib to help fighters run the city and saying they were not seeking to set up an Islamic emirate.

"There was an agreement by all factions to protect and ensure that essential services are continued," said an Ahrar al-Sham fighter who said he was speaking from Idlib during an interview conducted via the internet. He did not give his name.

But there are many questions over how groups that won Idlib in a joint operation will run the city.

"The rebels have certainly shown they can cooperate on the battlefield - that is not new - but the governance question has been much more difficult for them, particularly in the north, and particularly in Idlib province where Nusra has been asserting unilateral dominance," said Noah Bonsey, a senior analyst with International Crisis Group.

Mainstream rebel groups fighting Assad in southern Syria say they recently received increased support from his foreign enemies in response to a government offensive there.

Rami Abdulrahman, who runs the Observatory, said the death toll among Syrian government forces and personnel was much lower than among insurgents. An opposition activist in the area said the government had withdrawn personnel and anything sensitive to the state in apparent anticipation of losing the city.

(Reporting by Tom Perry and Mariam Karouny in Beirut, Suleiman al-Khalidi in Amman and Omar Fahmy in Cairo, and Humeyra Pamuk and Jonny Hogg in Turkey; Writing by Tom Perry; editing by David Stamp)