Syria said Tuesday it has received sophisticated new arms from Russia, including warplanes, and deployed them against jihadists, amid signs Moscow is readying a direct role in its ally's war.
The news came as a leading defence publication said Russia appeared to be preparing two bases for troops, but US Secretary of State John Kerry said the build-up may be purely defensive.
A senior military official told AFP Damascus had received new weaponry, including at least five fighter jets.
The deliveries came amid a Russian build-up in Syria, with US officials saying it had deployed 28 combat planes and begun drone flights.
The civil war has taken on a new dimension with the boosting of Moscow's military presence, raising deep concerns in Washington.
Robert Munks, editor of IHS Jane's Intelligence Review, said satellite imagery showed two sites near Latakia airport, in Assad's traditional heartland, "may be preparing to receive Russian forces".
Munks also noted a "substantial increase" in jets at Latakia airport, saying it pointed to a "rapid build-up of Russia's expeditionary force".
"With a significantly enhanced ground-attack capability, this represents a substantial step-change in Russia's combat ability."
The Syrian official said Damascus had also received "sophisticated military equipment to fight IS," including targeting equipment and precision-guided missiles.
The weapons had already been deployed against IS in the cities of Deir Ezzor and Raqa, the jihadist group's de facto capital.
"Russian weapons are starting to have an effect in Syria," the official said.
Kerry noted that while Russia had increased its deployment of warplanes to Syria, it appeared to be to defend existing bases.
"Yes, they have increased aircraft and there are certain kinds of aircraft there which, depending on what Russia's decision is about its long-term intentions, could raise questions," he said.
"But for the moment, it is the judgement of our military and most experts that the level and type represents basically force protection for their deployment to an airbase, given the fact that it is in an area of conflict."
Moscow has been an unwavering supporter of Assad in a conflict that has killed more than 240,000 since March 2011, insisting it would continue arms deliveries.
But its intentions are unclear.
US officials say Moscow has deployed 12 SU-24 attack aircraft, 12 SU-25 ground attack aircraft and four Flanker fighter jets.
They say there are also about 20 Russian combat and transport helicopters, and that Moscow is operating drones.
"They are not going to sit around," said Jeffrey White of the Washington Institute for Near East Policy.
"This kind of aircraft suggests that the Russians intend to exert their combat power outside of Latakia in an offensive role."
Fears have been growing of an accidental confrontation between Russian forces and the US-led coalition that has been carrying out air strikes against IS in Syria for more than a year.
US and Russian military officials held talks Friday aimed at avoiding such incidents after an 18-month freeze in their relations triggered by NATO anger over Moscow's role in Ukraine.
And Russian President Vladimir Putin agreed a deal Monday with Israeli Prime Minister Benjamin Netanyahu aimed at the same thing.
"A joint mechanism for preventing misunderstandings between our forces" was agreed to at talks in Russia, Netanyahu's office said.
Israeli forces have reportedly conducted several strikes in Syria on attempted Iranian arms transfers to Lebanese Shia movement Hezbollah, another Assad ally.
Russia is pushing for the coalition of Western and regional powers fighting IS to join forces with Assad against the jihadists.
Western and Gulf powers have long resisted any role for Assad in the fight against IS, insisting he must go for Syria to have any hope of peace.
Western diplomats suggest that Putin, isolated over the Ukraine crisis, is trying to switch the focus to Syria, ahead of a key address Monday to the UN General Assembly.
Anthony Cordesman, of the Washington-based Centre for Strategic and International Studies, said Russia was seeking to assert itself in the region.
"Putting aircraft means that everybody has to pay attention to Russia," he said. "Even if you fly a few demonstrative sorties, that will give you leverage."
Efforts to negotiate a solution to Syria's war have repeatedly failed, despite the enormity of a crisis that has uprooted millions, many of them seeking refuge in Europe.
The UN's envoy on Syria, Staffan de Mistura, held meetings this week in the latest bid to revive peace talks.
In July, he proposed an approach that would see Syrians taking part in "thematic" working groups on resolving the conflict.
The envoy met with the heads of these groups in the past two days, with the goal to "set the stage for a Syrian agreement to end the conflict," a statement said.
"This is the defining humanitarian challenge of our times," de Mistura said. "The Syrians deserve that we move faster towards a political solution."