Group of Seven leaders believe a window of opportunity for a political deal in Syria may be opening up that would eventually see President Bashar al-Assad step down, giving way to a new coalition government, officials familiar with the discussions at a G7 summit in the Bavarian Alps said.

The officials cautioned that no breakthrough was imminent and that Russia would need to be on board, but noted that Assad appeared to be increasingly on the defensive after setbacks for his army.

"This could be the opportunity to get a political deal. We need Russia for this," one of the sources said, adding that the only way for a transition would be for Assad to leave Syria.

Another G7 source said there was a feeling the situation in Syria could be on the verge of change, without giving details.

"There might be more we could do with the regime to work on a transition to different leadership in Syria," the source said, also adding that this would require Russian involvement. "(We should) work to break the political deadlock," the source said.

The sources did not say when or how they would advance such a transition, with no obvious alternative to Assad on the government side.

The view also conflicts with assessments from sources close to Damascus who see a protracted battle ahead and the crisis linked to broader issues including developments in Iraq and the outcome of a nuclear deal between Iran and world powers.

Iran is seen as essential to advancing any settlement to the four-year-old Syrian conflict.

The deputy leader of top Assad ally Hezbollah, which is backed by Iran, said last month insurgents would be unable to topple the Syrian government despite their recent gains and that Syria's allies, including Iran and Russia - would back Assad as long as needed.

The Syrian army and allied militia have lost large areas of land in the past two months to insurgents including the al Qaeda-linked Nusra Front and Islamic State, which has seized the central ancient city of Palmyra. – Reuters