MOSCOW -- Bashar Assad's surprise meeting with Vladimir Putin could signal that Russia ultimately seeks a political settlement after weeks of heavy airstrikes in Syria. But the terms of such an arrangement are uncertain, and questions remain about whether Moscow will seek the departure of its longtime ally or try for a power-sharing agreement.

In a further sign that a diplomatic push might be underway to end the four-year crisis, Russia announced Wednesday that Foreign Minister Sergey Lavrov and U.S. Secretary of State John Kerry had agreed to meet Friday in Vienna with their counterparts from Saudi Arabia and Turkey -- both firm Assad critics.

The Syrian president's visit to Moscow, his first known trip abroad since war broke out in 2011, was announced on Wednesday, the morning after it happened, and raised intense speculation about the two leaders' motives -- and a strong response from Washington.

"We view the red-carpet welcome for Assad, who has used chemical weapons against his own people, at odds with the stated goal by the Russians for a political transition in Syria," said White House spokesman Eric Schultz.

If nothing else, it underscored how emboldened the embattled Syrian leader has become in the wake of the Russian airstrikes that began on Sept. 30 and Iran's deployment of hundreds of ground forces to fight alongside Syrian government troops.

Russia says it is targeting militants, especially those of the extremist Islamic State group. But critics, including the U.S., say Moscow's military intervention props up Assad and is likely to fan the violence.

The oblique references Wednesday by both leaders to their meeting did little to shed light on their ultimate strategy.

In a statement, Putin said that along with fighting militants, Moscow believes that "a long-term settlement can only be achieved as part of a political process with the participation of all political forces, ethnic and religious groups."

In separate comments posted on the Syrian presidency's official Facebook page, Assad said the Russian military operation in Syria had helped to halt the spread of terrorism and that a political solution could only come after that threat was addressed.

"Terrorism which we see spreading today could have been more widespread and more harmful if it weren't for your decisions and steps," Assad told Putin in the remarks carried by Arab media. The threat of terrorism, he said, "obstructs" any political solution to the crisis.

Yet how long Russia is willing to continue its expensive and politically volatile airstrikes is open to question.

"Putin is not going to fight there forever," said analyst Dmitry Trenin of the Moscow Carnegie Center.

The Russian leader "needs a quick result after which he can announce a Russian military success," said Trenin, suggesting Russia would then put "the burden of the war" on Iran and Hezbollah.

"For Putin, it's important now to act not only as a military victor but as a political peacekeeper as well," Trenin said. "Putin is perfectly aware of the fact that Assad is going to have to hand over power or divide it. The Syria that existed before 2011 cannot be restored."

Others, however, said that in welcoming Assad, Putin was signalling that Russia would be uncompromising about Assad staying in power.

"It's a serious statement meaning that Russia will continue to support Assad to the bitter end," said Pavel Felgenhauer, an independent Russian military analyst. "There will be no political settlement in the sense that the West sees it. The opposition must surrender and stop fighting Assad and join him, and that is going to be the political solution."

Commentators on Syrian TV hailed the visit as an endorsement of Assad's legitimacy, reinforcing the notion that he must be part of a future political solution to the crisis.

"This lightning trip is a slap" to Assad's opponents, Syrian analyst Bassam Abdullah told state television channel Ikhbariyah, adding that it highlights the need for political meetings, not just military action.

Abdullah described the meeting as "intimate" and reflecting a common vision and values between the two leaders. "There is a clear strategic shift in the region ... and it is happening fast," he said.

Television footage showed Putin and Russia's foreign and defence ministers meeting with Assad, with the two leaders exchanging warm handshakes and smiles. Syrian government officials didn't say if anyone travelled with Assad and photos from the meeting didn't show a delegation accompanying the Syrian leader.

Assad said Russia's intervention was in line with international law and praised it as an effort to rid Syria and the region of terrorism.

Putin's spokesman, Dmitry Peskov, declined to comment on any specific outcome of the talks. Asked if Assad's departure from power was under consideration, he said it was too early to discuss political solutions while the fight against terrorism continued.

Since June, Russia has played with the idea of a political transition that would envisage setting up some sort of interim government, and has discussed the issue with the U.S., Saudi Arabia, the Syrian opposition and others.

Moscow's diplomatic efforts have brought no visible results so far, though Putin has insisted that a political solution for Syria remains his top goal despite Russia's military action. He recently met with Saudi officials, staunch critics of Assad and supporters of the rebels fighting against him. After Tuesday's meeting with Assad, he called the Saudi king to brief him about the talks before the foreign ministers' meeting in Vienna on Friday.

Moscow also has sought to alleviate the concerns of Turkey, a major economic partner and the second-biggest importer of Russian natural gas, which has been critical of Russia's intervention in Syria. Ankara also supports rebels fighting Assad.

Answering questions about Assad's visit to Moscow, Turkey's prime minister took a jab at the Syrian leader, reiterating his country's position that Assad shouldn't have a role in Syria's future.

"If only he could stay in Moscow longer, to give the people of Syria some relief. In fact, he should stay there so the transition can begin," Ahmet Davutoglu told reporters.

Davutoglu insisted that efforts to find a solution to the Syrian crisis should focus "not on a transition with Assad, but on formulas for Assad's departure."

------

Aji reported from Damascus. Associated Press writers Sarah El Deeb in Beirut, Darlene Superville in Washington and Jim Heintz and Kate de Pury in Moscow contributed to this report.

----

A previous version of this story has been corrected to show that the Syrian conflict began in 2011, not 2001.