BEIRUT — Syrian peace talks that opened less than a week ago after months of diplomatic maneuvering seemed on the verge of collapse Wednesday, as the chief United Nations mediator declared a “pause” and each side blamed the other for the setback.

The U.N. envoy, Staffan de Mistura, told reporters in a surprise announcement in Geneva that he was calling a “temporary pause” in the talks, which he said would be scheduled to resume Feb. 25.

Whether the talks would actually resume remained unclear amid recriminatory salvos from rival sides in the Syrian conflict and their foreign backers.

The Syria talks are considered the most robust diplomatic effort to date to end the Syrian conflict, now approaching its fifth year. U.S. officials, who support the talks, put part of the blame on Russian airstrikes backing a Syrian government offensive making significant advances in northern Aleppo province, which has long been split between opposition and loyalist forces.

“It is difficult in the extreme to see how strikes against civilian targets contribute in any way to the peace process now being explored,” John Kirby, a State Department spokesman, told reporters in Washington. “These attacks run counter not only to the desires of the Syrian people, who want to see this political process succeed, but also to the stated intentions of the Russians themselves” who joined with U.S. officials to help craft the peace-talk process.

Moscow said that its air power in Syria targets what it calls terrorists fighting to overthrow the Syrian government and not civilians.

The Russian airstrikes, which began Sept. 30, have been a major boost for the government of Syrian President Bashar Assad in the run-up to the Geneva talks. Russian air power has helped turn the tide of the war on several battlefronts, including the Aleppo area, leaving little incentive for Damascus to yield concessions demanded by the opposition. Russia said it has no intention of letting up.

“I see no reason to stop these airstrikes,” Russian Foreign Minister Sergey Lavrov told reporters in Oman.

The top Russian diplomat also took a verbal swipe at the main Syrian opposition bloc, the High Negotiations Committee, saying some members “are too spoiled by their sponsors,” apparent references to Saudi Arabia and Turkey, which have both backed armed insurgents seeking to topple Assad.

After the postponement in talks was announced, Riad Hijab, head of the opposition committee in Geneva, issued what amounted to an ultimatum, saying his group would not return absent some concessions from Damascus. The opposition bloc has demanded that the government and its Russian allies halt bombardment and that Damascus end sieges of rebel-held areas and release some prisoners.

“We will not return here unless we see something on the ground,” Hijab, a former Syrian prime minister who defected to the opposition, told reporters in Geneva.

The chief government negotiator, Bashar Jaafari, blamed the breakdown on the opposition, accusing it of a “childish” attempt to subvert the talks by demanding preconditions before they even began.

The suspension of talks was a significant setback for a process that began less than a week ago and was backed by major powers, including the U.S. and Russia. The various outside powers supporting the Geneva talks, known as the International Syria Support Group, are set to meet Feb. 11 to discuss the fate of the negotiations.

Tribune Newspapers' Tracy Wilkinson in Washington and special correspondent Nabih Bulos in Paris contributed.

patrick.mcdonnell@tribpub.com