MOSCOW — International mediator Lakhdar Brahimi said after talks with senior U.S. and Russian officials on Sunday that they had agreed it was still possible to find a political solution to the deepening crisis in Syria.

"The meeting was constructive and held in a spirit of cooperation. It explored avenues to move forward a peaceful process and mobilise greater international action in favour of a political solution to the Syrian crisis," Brahimi said in a statement issued at the end of all-day talks in Geneva.

Russian and U.S. diplomats met Sunday with Brahimi for more talks on the civil war in Syria, Russian Foreign Minister Sergey Lavrov said, adding that the Americans were wrong to see Moscow as softening its position.

Russia agreed to take part in the talks in Geneva, he said, on the condition there would be no demand for Syrian President Bashar al-Assad to step down, The Associated Press reported.

"We are not conducting any negotiations on the fate of Assad," Lavrov said Sunday. "All attempts to portray things differently are unscrupulous, even for diplomats of those countries which are known to try to distort the facts in their favor."

Also Sunday, Syrian rebels backed by radical Islamists captured a northern regimental command center of Assad's army, activists told Reuters.

Assad's forces hammered rebel units on the outskirts of Damascus as they tried to drive back opposition fighters rebels seeking to advance toward the embattled leader's seat of power.

Rebels have made a series of advances in recent weeks, partly due to help from radicals such as Jabhat al-Nusra, a group linked to al-Qaida in Iraq which has been excluded from a newly-formed rebel military command.

The Syrian Observatory for Human Rights told Reuters that Jabhat al-Nusra, which has called for the creation of an Islamic state in Syria, had participated in capturing the command center of the army's 111th regiment in the north of the country. It said about five soldiers were captured, while the commanding officer and some 140 of his men fled to another army site nearby.

Lavrov, the Russian foreign minister, met last week with Brahimi and U.S. Secretary of State Hillary Rodham Clinton in Dublin. Afterward, Clinton said the United States and Russia were committed to trying again to get both sides in the Syrian conflict to talk about a political transition. Clinton stressed that the U.S. would continue to insist that Assad's departure be a key part of that transition.

Russia and the United States have argued bitterly over how to address the conflict, which began with peaceful protests against Assad in March 2011 and escalated into a civil war that has killed an estimated 40,000 people. The U.S. has criticized Russia for shielding its closest ally in the Middle East, while Moscow has accused Washington of encouraging the rebels and being intent on regime change.

Russia's foreign minister said Sunday that after he agreed to a U.S. proposal to have his and Clinton's deputies "brainstorm" on Syria, the Americans began to suggest that Russia was softening its position.

"No such thing," Lavrov said. "We have not changed our position."

He urged the international community to come together and "with one voice" to demand a ceasefire, return U.N. observers in bigger numbers and begin a political dialogue. Lavrov repeated that Russia was not wedded to Assad but believed that only the Syrians have the right to choose their leaders.

Germany weighed in Sunday on the future of Assad's regime, with Federal Intelligence Service chief Gerhard Schindler saying it would not survive, although it was impossible to say how long it would hang on.

"Signs are increasing that the regime in Damascus is in its final phase," he was quoted as telling the Frankfurter Allgemeine Sonntagszeitung.

Addressing fears that Assad could use chemical weapons in a last-ditch effort to save his regime, Lavrov once again said the Syrian government has given assurances that it has no intention of ever using the weapons of mass destruction. He said the greatest threat is that they would fall into the hands of militants.

Lavrov said Russia takes seriously any rumor about Syria's chemical weapons and immediately clarifies the situation with the Syrian government, passing on any information to the Americans.