The West is floundering over how to respond to Syria's worsening civil war, especially after a leading Islamist rebel group pledged allegiance to Al-Qaeda, opposition officials and experts say.

The move by the Al-Nusra Front this week has complicated efforts by US and European officials to come up with unified strategy to support the rebels battling the regime of President Bashar al-Assad.

After pushing for weeks to lift a European embargo so arms could be supplied to the rebels, France and Britain have recently backed away from the initiative amid fears weapons could fall into the wrong hands.

Meeting in London this week, G8 foreign ministers said they were "appalled" by the spiralling violence in Syria but made no mention of supplying arms to the rebels.

The rebels' diplomatic representative in Paris, Monzer Makhous, said he was increasingly sceptical of the West backing up its promises of support for Syria's opposition with real action.

"There will never be any guarantees of where the arms will end up. And there will never be a total lifting of the European embargo," he said.

He admitted that Al-Nusra's declaration "is a problem", but said that the group was no excuse for the West's lack of a strategic plan to deal with the crisis in Syria.

"Frankly, I'm asking myself where are the West's great strategies? They have failed to see what is happening. At the start of the revolution there were no jihadists," he said, echoing other opposition figures who have blamed the West's failure to intervene more forcefully for the rise of Islamist groups.

The Al-Nusra Front has become a key spearhead in the rebel campaign in Syria, attracting many fighters, including foreigners, to join its ranks.

As well as arms, the West needs to do more to boost the opposition Syrian National Coalition on the diplomatic front, Makhous said.

"We have obtained Syria's seat at the Arab League. Now we want the seat at the UN but this seems extremely difficult.

"We can ask what all these meetings every few weeks are for," he said, pointing to another gathering of the "Friends of Syria" grouping of Western and Arab countries in Istanbul on April 20.

Still, Western diplomats have complained that the opposition coalition is divided and disorganised, making it difficult to provide support. Its leader Ahmed Moaz al-Khatib announced last month he will resign his post following the election of a provisional prime minister, Ghassan Hitto.

"We want to know who we are dealing with," French Foreign Minister Laurent Fabius recently complained of the frequently shifting rebel leadership.

"The main problem today is not a question of arms, it's a question of the rebel structure on the ground," said Florence Gaub, a researcher at the NATO Defense College in Rome.

She said it was unlikely a political solution to the conflict would be possible this year and that the stalemate between the rebels and Assad's forces would continue.

More than 70,000 people have died since March 2011 protests against Assad sparked a crackdown, armed uprising and eventually a full-blown civil war. But despite the mounting death toll, neither side has been able to claim a decisive advantage on the ground.

"The rebels believe they can tire Bashar's regime, but Damascus's military strategy is to keep hitting them until they stop. Even if they (the rebels) have more sophisticated arms and destroy planes, the regime's response will be even stronger," Gaub said.