Talks on Syria’s civil war among Russia, the U.S. and Middle East powers ended without result amid a warning in Moscow that the risk of conflict is increasing between the two former Cold War rivals.

Russian Foreign Minister Sergei Lavrov left Lausanne, Switzerland, for Moscow after about four hours of talks on Saturday, saying the sides had discussed certain “ideas” for advancing a resolution of the 5 1/2 year war and agreed to pursue efforts again soon.

The negotiations over the deepening conflict in Syria marked a bid to pull back from a diplomatic collision over the Russian-backed siege of Aleppo by President Bashar al-Assad’s forces in a standoff that has brought ties between Russia and the U.S. to a recent low.

Secretary of State John Kerry met with Lavrov and their counterparts from Saudi Arabia, Qatar, Iran, Jordan, Iraq, Egypt and Turkey, aiming to restore a truce to deliver aid supplies to Aleppo and other besieged areas. Kerry said there was no agreement on any concrete steps to halt the violence, the Associated Press reported. He said some ideas under discussion could prove helpful to restarting the peace process.

The U.S. had said the meeting was aimed at getting Russia and Syria to halt their bombing of Aleppo, the country’s former commercial capital, where 250,000 civilians are trapped.

The conflict, which has killed more than 300,000 people and displaced millions, is at increasing risk of morphing into a proxy war between Russia, the U.S. and regional powers.

The offensive at Aleppo has ignited tensions in the UN Security Council, raised the possibility of more sanctions against Russia and prompted comments on Russian state media about “a nuclear dimension” in any confrontation with the U.S. The Kremlin has rejected Kerry’s accusation that it committed war crimes, as the U.S. and its allies consider giving rebels more weapons to fight Assad.

The talks marked the first attempt at diplomacy since a U.S.-Russian cease-fire collapsed in September after only a week. The resumption of hostilities kicked off an escalating spiral of accusations, with Kerry saying last week Russia and Assad were terrorizing civilians.

The head of the Russian lower house of parliament’s defense committee, Vladimir Shamanov, warned in an interview on state TV broadcast Saturday that “the likelihood is increasing” of a direct military clash between Russia and the U.S.

A senior Russian official said Moscow hadn’t ruled out the possibility of a strike by the U.S.-led coalition on Russian service members stationed in Syria. Such risks remain high while the offensive to capture Aleppo continues, said the official, who asked not be named because of the sensitivity of the matter. All U.S.-driven initiatives such as safe corridors for fighters will delay the inevitable and the city should fall into Assad’s hands by the end of the year, he said.

The Russian and Syrian bombing of Aleppo, once a city of 2.3 million near the northwestern border with Turkey, has eased only slightly after a punishing wave of strikes from late September to early October. Opposition activists said the attacks killed hundreds of civilians and destroyed hospitals, sparking international condemnation.

Assad said Aleppo is an important “springboard” in the campaign against the rebels, according to an interview in English published Friday in Russian newspaper Komsomolskaya Pravda. The remaining rebel stronghold of Idlib, about 60 kilometers (35 miles) southwest of Aleppo, would be the next objective, he said. If taken, it would hand the government control of all major urban centers in Syria except for Raqqa, the capital of the self-declared caliphate of Islamic State.

Meanwhile, the U.S. is examining ways to provide more weapons to opposition fighters via allies such as Turkey and Saudi Arabia, according to a U.S. administration official who spoke on condition of anonymity to discuss internal deliberations. Kerry will head to London for talks with key European allies on Sunday.