By Alistair Bell and Tom Perry

WASHINGTON/MOSCOW (Reuters) - U.S. President Barack Obama warned Russia on Friday that its bombing campaign against Syrian rebels will suck Moscow into a "quagmire," after a third straight day of air raids in support of President Bashar al-Assad.

At a White House news conference, Obama frequently assailed Russian President Vladimir Putin, who he accused of acting out of a position of weakness to defend a crumbling, authoritarian ally.

Friday prayers were canceled in insurgent-held areas of Syria's Homs province hit by Russian warplanes this week, with residents concerned that mosques could be targeted, according to one person from the area.

Putin's decision to launch strikes on Syria marks a dramatic escalation of foreign involvement in a more than four-year-old civil war in which every major country in the region has a stake.

It also gives fuel to domestic critics of Obama who say his unwillingness to act on Syria has allowed Moscow to stage its biggest show of force in the Middle East in decades.

But the U.S. president warned that Russia and Iran, Assad's main backer in the Muslim world, have isolated the majority of Syrians and angered their Sunni Muslim neighbors.

"An attempt by Russia and Iran to prop up Assad and try to pacify the population is just going to get them stuck in a quagmire and it won't work," Obama said.

The Syria campaign is the first time Moscow has sent forces into combat beyond the frontiers of the former Soviet Union since the USSR's disastrous Afghanistan campaign of the 1980s, a bold move by Putin to extend Russia's influence beyond its neighborhood.

It comes at a low point in Russia's relations with the West, a year after the United States and EU imposed financial sanctions on Moscow for annexing territory from Ukraine.

Obama on Friday offered to work with Russia to bring peace to Syria, but he took several digs at Putin, with whom he has a frosty relationship. A meeting between the two at the United Nations this week seems to have done little to produce a thaw.

Story continues

"Mr. Putin had to go into Syria not out of strength, but out of weakness because his client Mr. Assad was crumbling and it was insufficient for him to send arms and money," Obama said.

He played down international support for Moscow's strategy, saying it paled in comparison to the number of countries backing U.S. air raids on Islamic State.

The U.S. president has been deeply reluctant to use more military force in Syria, after America's experience of long wars in Iraq and Afghanistan.

Obama also hit back at critics who say his Syria strategy of bombing Islamic State and supporting moderate rebels is failing.

"I hear people offering up half-baked ideas as if they are solutions or trying to downplay the challenges involved in the situation. What I'd like to see people ask is, specifically, precisely, what exactly would you do and how would you fund it and how would you sustain it? And typically, what you get is a bunch of mumbo jumbo," he said.

BOMBING INSURGENTS

Russia bombed Syria for a third day in a row on Friday, mainly hitting areas held by rival insurgent groups rather than the Islamic State fighters it said it was targeting and drawing an increasingly angry response from the West.

The U.S.-led coalition that is waging its own air war against Islamic State called on the Russians to halt strikes on targets other than Islamic State.

In Syria, Islamic State is one of many groups of insurgents fighting against Assad. Washington and its Western and regional allies say Russia is using it as a pretext to bomb other groups that oppose Assad. Some of these groups have received training and weapons from Assad's foreign enemies, including the United States.

Also on Friday, Putin held frosty talks with France's Francois Hollande in Paris, the Russian leader's first meeting with a Western leader since launching the strikes two days after he gave an address to the United Nations making the case to back Assad.

Warplanes were seen flying high above an area of Syria's Homs province where Friday prayers were canceled. The area is held by anti-Assad rebels, but has no significant presence of Islamic State fighters.

"The streets are almost completely empty and there is an unannounced curfew," said a resident, speaking from the town of Rastan which was hit in the first day of Russian air strikes.

Islamic State also canceled prayers in areas it controls, according to activists from its de facto capital Raqqa.

A Russian air strike on Thursday destroyed a mosque in the town of Jisr al-Shughour, captured from government forces by an alliance of Islamist insurgents earlier this year, activists said.

Western countries and Russia say they have a common enemy in Islamic State. But they also have very different allies and opposing views of how to resolve a war that has killed at least 250,000 people and driven more than 10 million from their homes.

Washington and its allies oppose both Islamic State and Assad, blaming him for attacks on civilians that have radicalized the opposition and insisting that he has no place in a post-war settlement.

Lebanese sources have told Reuters that hundreds of Iranian troops have also arrived in recent days in Syria to participate in a major ground offensive alongside government troops and their Lebanese and Iraqi Shi'ite militia allies.

Syria's foreign minister said the U.S.-led coalition's campaign against Islamic State was bound to fail.

"Air strikes are useless unless they are conducted in cooperation with the Syrian army, the only force in Syria that is combating terrorism," Walid al-Moualem said in a speech to United Nations General Assembly in New York.

Moscow said on Friday its latest strikes had hit 12 Islamic State targets, but most of the areas it described were in western and northern parts of the country, while Islamic State is mostly present in the east.

The Russian Defence Ministry said its Sukhoi-34, Sukhoi-24M and Sukhoi-25 warplanes had flown 18 sorties hitting targets that included a command post and a communications center in the province of Aleppo, a militant field camp in Idlib and a command post in Hama.

A Defence Ministry official, Igor Konashenkov, later told Russian news agencies the Air Force had conducted 14 flights in Syria on Friday and made six strikes against Islamic State targets. The ministry was not immediately available to comment on the apparent discrepancy with the earlier figures.

(Additional reporting by Andy Sullivan, Roberta Rampton and David Alexander in Washington, Elizabeth Pineau in Paris, Vladimir Soldatkin in Moscow, David Dolan in Istanbul and Orhan Coskun in Ankara; writing by Alistair Bell, editing by G Crosse)