Signs of an ongoing Russian military buildup in Syria have drawn U.S. concerns and raised questions of whether Moscow plans to enter the conflict. President Vladimir Putin has been coy on the subject, saying Russia is weighing various options, a statement that has fueled suspicions about the Kremlin's intentions.

Observers in Moscow say the Russian maneuvering could be part of a plan to send troops to Syria to fight the Islamic State group in the hope of fixing fractured ties with the West. They warn, however, that Putin would likely find it hard to sell his idea to a skeptical U.S. and risks potentially catastrophic repercussions if he opts for unilateral military action in Syria.

By playing with the possibility of joining the anti-IS coalition, Putin may hope to win a few key concessions. His main goal: the lifting of Western sanctions and the normalization of relations with the United States and the European Union, which have sunk to their lowest point since the Cold War amid the Ukrainian crisis. In addition, the Russian leader may be angling to make the West more receptive to Moscow's involvement in Ukraine, while retaining influence in Syria.

Early this summer, the Kremlin put forward a peace plan for Syria that envisions enlisting Syrian government forces and Iran in the anti-IS coalition. A few rounds of negotiations with the Americans and Saudis have brought no visible results, and now Moscow appears to be testing the water for a next move: beefing up its military presence in Syria.

While Putin said Friday there is no talk "just yet" about Russian troops joining the fight against the Islamic State, he seemed to keep the door open for the possibility, saying "we are looking at various options." The Russian leader is set to attend the United Nations General Assembly later this month, and some analysts say a proposal to deploy troops to Syria could be the focal point of his visit.

Since the Soviet times, Russia has had close political and military ties with Syria, which hosts a Russian navy facility in the Mediterranean port of Tartus intended to service and supply visiting ships. While the Soviet-era facility has just a couple of floating piers along with a few rusting repair shops and depots, it has symbolic importance as the last remaining Russian military outpost outside the former Soviet Union.

Moscow has staunchly backed Syrian President Bashar Assad throughout the nation's 4 ½-year civil war, providing his regime with weapons and keeping military advisers in Syria. Putin said again Friday that Russia is providing the Syrian military with weapons and training.

Rami Abdurrahman, the head of the Britain-based monitoring group Syrian Observatory for Human Rights, said there have been reports since mid-August of Russian troops in the capital's airport and another airport in the coastal city of Latakia.

"We don't know if they are troops or transporters of weapons and ammunition," he said, noting an increase in the flow of Russian weapons arriving in Syria since July.

"The fact that (military cooperation) is not new is one thing, but there is a noticeable increase," said Abdurrahman, who has a large network of activists on the ground in Syria helping him monitor the situation.

U.S. Secretary of State John Kerry signaled Washington's concern in a phone call with his Russian counterpart Sergey Lavrov over the weekend. The State Department said that Kerry made it clear that if reports of an imminent Russian military build-up in Syria were accurate, "these actions could further escalate the conflict, lead to greater loss of innocent life, increase refugee flows and risk confrontation with the anti-ISIL Coalition operating in Syria."

According to a U.S. defense official, who spoke on condition of anonymity because they weren't authorized to publicly discuss the sensitive issue, the U.S. has seen an increasing number of Russian transport planes seeking diplomatic approval for flights into Syria. He said it's not clear what is in the aircraft or what their purpose is.

Greek Foreign Ministry spokesman Constantinos Koutras said the U.S. has asked Greece to cancel overflight permission given to Russia September 1-24, for flights headed to Syria. He said they are examining the request.

The U.S. officials, who declined to be identified because they weren't authorized to speak publicly about the issue, said they also have seen the movement of some prefabricated housing in Syria, although they haven't seen any troops moving in or becoming involved in actual combat activities, as some media reports suggested.

Sergei Karaganov, the founder of the Council on Foreign and Defense Policy, a leading association of Russian political experts, said that Russia was considering the possibility of joining the anti-IS coalition, but the West so far has been unwelcoming. "They are reluctant to accept proposals from Putin, whom they want to contain," he said.

Karaganov, who has good connections among the Russian officials, said he doesn't expect Russia to opt for unilateral military action in Syria if it gets the cold shoulder from the U.S. and its allies. "It would involve enormous risks," he said.

Igor Korotchenko, a retired colonel of the Russian military's General Staff who is now editor of the National Defense magazine, also said that while Russia has supplied Assad's government with weapons, it has no intention to send its troops to Syria.

"Russia will not send its troops to the Middle East, it's absolutely excluded," he said. "It's the U.S. problem. Russia will not pay for that with its soldiers' lives."

Alexander Golts, an independent military analyst, said Putin sees joining the anti-IS coalition as a chance to reach rapprochement with the West. "Russia has found itself in isolation, which has been increasingly felt," he said.

He said the latest reports about the movements of troops and military cargos to Syria appeared to demonstrate Moscow's readiness to join the coalition, falling short of a big-size deployment.

Pavel Felgenhauer, a Moscow-based analyst who specializes in military and security issues, said that the apparent increase in the Russian presence in Syria could be part of Kremlin efforts to raise the pressure on the U.S. to accept Putin's plan.

"Such a coalition ... would allow Assad's regime to survive and allow Russia to maintain its presence in the Middle East," he said.

If Russia ends up sending its military contingent to Syria, it will likely include a few combat jets along with support personnel and some troops to guard them, Felgenhauer said. Staying away from ground action would allow Russia to avoid any significant losses.

Alexei Malashenko, a Middle East expert with the Carnegie Endowment's Moscow office, was skeptical, saying that Putin's apparent plan to use Syria to improve ties with the West will be unlikely to succeed.

He warned that if Russia fails to strike a deal with the U.S. and tries to do it alone alongside Assad's forces, it would further damage its relations not just with the U.S. but regional powers. It will also likely trigger a negative public response, providing a painful reminder of the botched Soviet war in Afghanistan.

"It will not be received with joy here in Russia; everyone will compare it to Afghanistan," he said. "If they do it, it would be a very stupid thing. It's very simple to get in, but it could be quite difficult to get out."

Malashenko also warned that deploying Russian soldiers to fight the IS would draw risks of retaliation and raise the terror threat for Russia.

While launching unilateral action would be extremely risky, it's difficult to predict how Putin will act if his offer of joint action against the IS is rejected by Washington, Malashenko said.

"Putin is unpredictable, and he is very emotional," he said.

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Lolita Baldor in Washington, Sarah El Deeb in Cairo and Elena Becatoros in Athens contributed to this report.