Rebels killed 28 soldiers in Syria's northwestern battlefields Thursday, a watchdog said, as the regime launched new air strikes in what is seen as a desperate attempt to reverse opposition gains.

The fresh fighting came as the main opposition Syrian National Council hit back at US warnings of rising Islamic extremism among Syria's rebels, saying the West and its partners were to blame for rising radicalisation.

And China, amid stalled international peace efforts, said it had made "constructive new suggestions" to end the bloodshed during talks with UN-Arab League peace envoy Lakhdar Brahimi.

The 28 soldiers were killed in attacks on three army checkpoints in northwestern Idlib province, on the main road from Damascus to the embattled city of Aleppo, the Britain-based Syrian Observatory for Human Rights said.

Five rebels were also killed in the fighting near the city of Saraqeb in Syria's northwest, which has become a key battleground after rebel forces seized the town of Maaret al-Numan on the Damascus-Aleppo road early last month.

Thursday also saw helicopter gunships strafing a district of Damascus as warplanes pounded rebel bastions in the capital's suburbs and in Idlib, the Observatory said.

At least three warplane raids were conducted in the northern Damascus suburb of Harasta, home to some of the rebel Free Syrian Army's best organised fighters, as on the other side of the city gunships hit the neighbourhood of Al-Hajar Al-Aswad, it said.

President Bashar al-Assad's forces have launched a wave of intensive air strikes this week that analysts say are a response to opposition gains and aimed at "terrorising" local communities.

"They are trying to make the civilian population so angry and so scared that it will not be possible for the rebels to find safe havens," said Riad Kahwaji, head of the Dubai-based Institute for Near East and Gulf Military Analysis.

But Kahwaji, said the strategy may be having the opposite effect by fuelling public fury.

"You are only angering the population and making them ask for blood in return."

Clashes also raged in the northern commercial hub of Aleppo, the Observatory said, and elsewhere in Idlib, where FSA forces backed by the Islamist Al-Nusra Front continued their siege of the Wadi Daif army base.

Violence on Wednesday killed at least 152 people across Syria, including 58 civilians, said the Observatory.

It says more than 36,000 people have now been killed since the uprising against Assad's regime broke out in March 2011 and evolved into an armed civil conflict.

US Secretary of State Hillary Clinton on Wednesday warned against efforts by Islamic extremists to "hijack" Syria's revolution, but the head of Syria's main opposition Syrian National Council said the lack of an international response to the conflict was to blame.

"The international community is responsible, through its lack of support for the Syrian people, for the growth of extremism in Syria," SNC director Abdel Basset Saida told AFP.

"The international community should criticise itself, and ask itself: What did it give the Syrian people? How has it helped the Syrians to stop the regime's crazy killing?" he said.

Deep divisions over how to deal with Assad -- China and Russia have repeatedly used their veto in the UN Security Council to block resolutions aimed at putting more pressure on the Syrian leader -- have stymied international efforts to address the conflict.

But China said Thursday it had made "constructive new suggestions" to end the bloodshed, including a phased region-by-region ceasefire and the formation of a transitional government.

In an apparent attempt to position China at the heart of efforts to solve the issue, the foreign ministry gave a detailed account of proposals it made to Brahimi during his visit this week.

"China's position on the Syrian issue is consistent. The new proposal is an extension of China's efforts to push forward a political resolution of the Syrian issue," ministry spokesman Hong Lei told a regular briefing.

Brahimi, who visited Moscow and Beijing this week in bid to revive peace efforts after a failed ceasefire during last weekend's Eid al-Adha Muslim holiday, is due to present new proposals for resolving the conflict to the UN Security Council later this month.

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