Sieges on cities across Syria must be lifted and air strikes on civilian areas must stop if the opposition is to resume talks in Geneva as planned on February 25, a top opposition leader said on Wednesday.

"Before we go on February 25 those measures should be implemented in reality on the land," Riad Hijab, chief coordinator of the opposition High Negotiations Committee (HNC), told reporters in London.

As well as the lifting of sieges on cities and towns, he listed letting aid through and stopping air strikes on civilian areas as conditions for attending the talks.

He was speaking through a translator after a meeting with British Foreign Secretary Philip Hammond.

Syrian peace talks were suspended this month until February 25 after UN envoy Staffan de Mistura said more work was needed to make progress.

Syria's nearly five-year-old conflict has claimed 260,000 lives and displaced half the population.

The city of Aleppo is facing a major operation by government forces backed by Russian air raids.

The UN has warned that 300,000 people in the east of the city could be cut off from humanitarian aid if government forces encircle the area, a tactic used by the regime to devastating effect against other former rebel bastions.

Following the meeting with Hijab, Hammond again urged a "genuine commitment" to peace talks and a political transition in Syria away from President Bashar al-Assad.

"Russia and the regime are deliberately targeting the opposition and thus strengthening Daesh," he added in a statement.

"Their actions against civilian populations and infrastructure are in breach of international humanitarian law."