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The rise of the Islamic State group (IS, formerly known as ISIS), could lead to a truce between Syrian government and opposition forces, suggested the UN envoy to Syria, Staffan de Mistura, on Tuesday.

The emergence of IS and al-Nusra Front, al-Qaeda's Syrian franchise, had made pro-Assad and Free Syrian Army-affiliated forces question whether the war was "giving an opportunity to someone else," de Mistura told the BBC from the Syrian city of Homs.

De Mistura is trying to push through a plan for a "freeze" to fighting in certain areas of the country.

Local truces could help enable humanitarian assistance to reach in Syria in need.

"A peace deal would be ambitious and delusionary," De Mistura said. "But I do have an action plan, and the action plan starts from the ground - stop the fighting, reduce the violence."

I do have an action plan, and the action plan starts from the ground - stop the fighting, reduce the violence. - Staffan de Mistura, UN envoy to Syria

Syrian President Bashar al-Assad appeared to welcome De Mistura's plan. His office on Monday said he was "ready to consider" a localised truce in Aleppo, the historic city that has been fought over since July 2012.

Any talk of a deal with Assad is likely to be rejected by the Syrian opposition.

Hadi al-Bahra, the head of the Syrian National Coalition, is pushing for the remit of the US-led coalition strikes against teh Islamic State group and al-Nusra to be widened to include strikes against Assad forces.

"The coalition is fighting the symptom of the problem, which is IS, without addressing the main cause, which is the regime," al-Bahra said in an interview with The Guardian newspaper on Monday. 

"The whole operation has been confused. Airstrikes will not be able to win the battle against extremism. You have to defeat IS on the ground," he said.

Bahra is in the UK attempting to win support for the Syrian opposition, and the military units affiliated to it, who fight under the umbrella of the "Free Syrian Army".

After meeting Bahra on Monday, UK Foreign Secretary Philip Hammond said that Britain would make "a significant contribution" to equip and train the FSA.

"We are providing non-lethal equipment and the UK expects to make a significant contribution to the US-led 'Train and Equip' programme," Hammond said.