U.S. officials say Syria's military remains loyal despite recent high-profile defections, while the opposition remains fragmented and unable to attack as a unified force, indicating a long, protracted conflict to come.

The Syrian regime is maintaining troop loyalty by keeping pay cheques coming even as food and fuel run out for the rest of the country, according to U.S. intelligence officials who briefed reporters on the Syrian conflict Tuesday. The officials spoke on condition of anonymity to provide the sensitive information.

The regime has also used militias to attack Sunni neighbourhoods instead of involving the mostly Sunni army, they said.

Officials say the opposition has learned effective guerilla-like tactics, but is still too disorganized to hold territory, which will likely produce a continuing "see-saw" battle between the two sides.

Syria's elite Republican Guard forces clashed with rebels just outside Damascus Tuesday in some of the most intense fighting involving the special forces guarding the capital since an uprising against President Bashar Assad's regime began last year, activists said.

The clashes erupted near Republican Guard housing compounds and bases in the suburbs of Qudsaya and Hammah, about eight kilometres from central Damascus, the Britain-based Syrian Observatory for Human Rights said. It said at least six people were killed in the fighting.

The Republican Guard, which is commanded by Assad's younger brother Maher, is tasked with protecting the capital, the seat of the regime's power.

The fighting suggested growing boldness by rebels as the more than 15-month effort to oust Assad turns increasingly militarized.

Clashes erupt regularly in the suburbs between troops and rebels, but Assad's forces have firm control of Damascus and it is very rare for fighting to take place near the Republican Guard bases. It was unclear what prompted the clashes or how close they were to the heavily guarded compounds.

Wave of defections

Assad has faced an embarrassing wave of defections in recent days, including several senior officers and soldiers who fled to neighbouring Turkey. Although high-level defections appear to be increasing, Assad's inner circle has remained largely intact.

State-run news agency SANA confirmed clashes in the area, claiming that dozens of gunmen attacked civilians and troops in Hammah. SANA said troops fought the attackers, including foreign fighters, killing dozens of them and detaining others.

Troops confiscated automatic rifles, rocket-propelled grenade launchers and large amounts of ammunition, according to SANA.

The Observatory and the Local Coordination Committees, another activist network, said troops stormed the Damascus neighbourhood of Barzeh with armoured personnel carriers, killing at least one person. Elsewhere in Syria, the LCC and the Observatory reported intense shelling in the eastern city of Deir el-Zour that killed at least five people.

Assad has promised reforms and named a new government last week, but it is headed by a key loyalist and the foreign, defence and interior ministers kept their jobs.

Syria's new cabinet was sworn in front of Assad on Tuesday, Syrian TV reported. The new government is headed by Riad Farid Hijab, a former agriculture minister and a member of the ruling Baath Party.

The opposition, which boycotted the parliamentary elections in May, accuses Assad of trying to buy time and strengthen his power even as international condemnation over the regime crackdown grows.

Further unrest feared

As Syria veers toward civil war, fears are mounting that the violence could ignite regional unrest. Syrian activists say that more than 14,000 people have been killed since the conflict began in March 2011.

In Brussels, NATO's governing body met Tuesday and discussed the shooting down of a Turkish military plane by Syria last week.

The head of the NATO military alliance called the downing of the jet unacceptable after Turkey briefed NATO's North Atlantic Council on the incident. The talks were held under Article 4 of NATO's founding treaty, which allows a NATO member to request consultations if its security has been threatened.

NATO Secretary-General Anders Fogh Rasmussen said the alliance condemned the Syrian attack "in the strongest terms" and expressed solidarity with Turkey but did not speak of any possible armed action against Syria.

"It's my clear expectation that the situation won't continue to escalate," Fogh Rasmussen told reporters after Tuesday's meeting. "What we have seen is a completely unacceptable act and I would expect Syria to take all necessary steps to avoid such events in the future."