A senior military official told AFP Damascus had received a fresh batch of arms, including at least five fighter planes, while a monitoring group said there had been a marked increase in regime attacks on the Islamic State group.

The deliveries came amid a rapid Russian military build-up in Syria, with US officials saying Moscow had deployed 28 combat planes and begun drone flights in the country.

The war in Syria has taken on a new dimension in recent days as Moscow has moved to boost its military presence there, raising deep concerns in Washington.

The Syrian military official said the new warplanes had arrived Friday along with reconnaissance aircraft at a military base in Latakia province, the traditional heartland of President Bashar al-Assad's regime.

Damascus had also received "sophisticated military equipment to fight IS," including targeting equipment and precision-guided missiles, the official said, speaking on condition of anonymity.

The new weapons had already been deployed against IS in the cities of Deir Ezzor and Raqa, the jihadist group's de facto capital in Syria.

"Russian weapons are starting to have an effect in Syria," the official said.

- 'Not going to sit around' -

The Syrian Observatory for Human Rights said new Russian equipment was being put into action, with at least 38 IS fighters killed Monday in air strikes in jihadist-held towns in central Syria.

"The number of raids is growing and the strikes are more precise after the Syrian air force received arms and more efficient planes from Moscow," Observatory director Rami Abdel Rahman told AFP.

Moscow has been an unwavering supporter of Assad during a conflict that has seen more than 240,000 killed since March 2011, insisting it would continue arms deliveries.

But Russia's intentions have been unclear in recent days as it deployed a range of new weaponry and troops to its airbase near Latakia.

On Monday, US officials said Moscow had deployed 12 SU-24 attack aircraft, 12 SU-25 ground attack aircraft and four Flanker fighter jets.

They said there were also about 20 Russian combat and transport helicopters at the base and that Moscow was operating drone flights.

"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."

The deployments have raised fears of an inadvertent 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.

After an 18-month freeze in military relations triggered by NATO anger over Moscow's role in the Ukraine crisis, US and Russian military officials held talks Friday aimed at avoiding unintended incidents in Syria.

In another potential sign of an increasing Russian role, President Vladimir Putin agreed a deal with Israeli Prime Minister Benjamin Netanyahu on Monday aimed at avoiding incidents in Syria.

- Bid to revive talks -

"A joint mechanism for preventing misunderstandings between our forces" was agreed to at talks in Russia, Netanyahu's office said.

Israeli forces have reportedly carried out several strikes in Syria on attempted Iranian arms transfers to Lebanese Shiite movement Hezbollah, an Assad ally.

The deployments come as Russia pushes 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 by the West over the crisis in Ukraine, is trying to switch the focus to Syria, ahead of a key address to the UN General Assembly on September 28.

Anthony Cordesman, of the Washington-based Centre for Strategic and International Studies, said recent Russian moves were also aimed at asserting Moscow's role 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 find a negotiated solution to Syria's war have repeatedly failed, despite the enormity of a crisis that has forced millions from their homes, 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, his office said Tuesday.

In July, he proposed a fresh 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."

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