Sieges on towns and 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.

Pro-Kremlin Chechen leader Ramzan Kadyrov has claimed spies from his region of Russia are being sent to infiltrate the Islamic State group and assist Moscow in its bombing campaign in Syria.

Kadyrov, who has led the North Caucasus region of Chechnya since 2007, made the claims in a preview of a documentary set to air on state television on Wednesday.

The Kremlin on Tuesday issued a rare rebuke to German Chancellor Angela Merkel a day after she criticized Russian air strikes in Syria.

In Ankara on Monday, Merkel -- referring to air strikes including those carried out by Russia -- said "we are horrified in the face of this human suffering."

Turkey said on Sunday it would not abandon thousands of Syrians stranded on its border after fleeing a major Russian-backed regime offensive, as aid agencies warned of a "desperate" situation.

Tens of thousands of people, including many women and children, have been uprooted as pro-government forces backed by intense Russian anti-rebel air strikes advance near Syria's second city Aleppo.

Up to 20,000 Syrians were stranded on the Turkish border Friday after fleeing a major Russian-backed regime offensive near Aleppo where a new humanitarian disaster appeared to be unfolding.

Tens of thousands of civilians have joined an exodus to escape fierce fighting involving government forces who severed the rebels' main supply route into Syria's second city.

The U.N. Security Council will meet Friday for consultations with U.N. special envoy Staffan de Mistura on the breakdown in Syria peace talks, diplomats said Thursday.

Mistura will report to ambassadors of the 15 Security Council members on the decision to suspend the talks in Geneva.

Tens of thousands of Syrians were reported to have fled their homes on Thursday as regime troops pressed a major Russian-backed offensive around second city Aleppo.

Turkey's Prime Minister Ahmet Davutoglu, speaking at a donors conference in London, warned that up to 70,000 people were headed towards his country to escape the fighting.

"Military escalations" by the Syrian regime led to the suspension of U.N.-brokered peace talks in Geneva this week but there is still a "glimpse of hope," Germany's foreign minister said Thursday.

After meeting King Salman and other leaders in Saudi Arabia, Frank-Walter Steinmeier said the talks broke down "because of the military escalations by the regime".

U.S. Secretary of State John Kerry demanded Thursday that Russia stop bombing the Syrian opposition, implicitly blaming Moscow for the collapse in peace talks.

Speaking in London ahead of a conference on the Syrian humanitarian effort, Kerry said he had called Russian Foreign Minister Sergei Lavrov for a "robust" discussion.

Turkey on Thursday said that it had refused to allow a Russian reconnaissance plane to overfly its territory near Syria, citing a disagreement over the flight plan, as relations between the two countries hit a post-Cold War low.

The Turkish foreign ministry's statement came a day after Russia accused Turkey of breaching the Open Skies treaty by refusing the plane access.