Russia has deployed 28 combat aircraft to its base in Syria and started flying surveillance drones as part of a steady military build-up, the New York Times said, citing US officials.
A total 12 Su-25 Frogfoot ground-attack planes and 12 Su-24 Fencers had arrived at the Russian base in Latakia, confirming Moscow's latest move in the war-ravaged country.
US officials claimed four Flanker air-to-air fighters were earlier deployed at the base. Other military assets at the base include 15 Russian Hip transport and Hind attack helicopters. Ground forces include nine T-90 tanks and more than 500 marines, which is at least twice the number from last week.
The base is also protected by several SA-22 surface-to-air, anti-aircraft systems. "The equipment and personnel just keep flowing in," according to US defence authorities.
Despite all the preparation, Moscow has yet to commence military operation in Syria.
US Secretary of State John Kerry said over the weekend that he welcomed Russia's involvement in fighting the Islamic State militants in Syria. Kerry made the statement after US and Russian defence chiefs had agreed on several issues to avoid "accidental interactions."
Kerry said the worsening refugee crisis highlighted the need to find a compromise that could lead to a political change in the country.
John Kirby, a State Department spokesman, on the other hand, said this was a conditional welcome.
"If what they're doing is, in fact, propping up the Assad regime, then that's an entirely different issue altogether, because it is the Assad regime that has been a magnet for extremists inside Syria," he said.
Syrian President Bashar al-Assad has long been regarded as Russia's strongest ally in the region.
Meanwhile, Israeli Prime Minister Benjamin Netanyahu told a news conference that after talks in Moscow with Russian President Putin, they had ''agreed on a mechanism to prevent such misunderstanding.''
"There was readiness to make sure that whatever Russia's intentions for Syria, Russia will not be a partner in extreme actions by Iran against us," the prime minister said, according to Reuters.
The civil war in Syria has killed an estimated 250,000 people and many continue to flee their homes.
