NEW YORK: Russiais building a military base in Syrian president Bashar al-Assad's heartland, according to American intelligence officials, in the clearest indication yet of deepening support from Moscow for the embattled regime.
The anonymous officials say Russia has set up an air traffic control tower and transported prefabricated housing units for up to 1,000 personnel to an airfield serving the Syrian port city of Latakia.
Russia has also requested the right to fly over neighbouring countries with military cargo aircraft throughout this month, according to the reports.
The claims, which will raise fears that Russia is planning to expand its role in the country's civil war, will ratchet up tensions between Moscow and Washington over the future of Syria and its brutal ruler.
Russia has been a key supporter of Mr Assad during the past four years of civil war, using its UN Security Council veto to ease international pressure on his embattled regime.
On Friday Barack Obama met with King Salman of Saudi Arabia to repeat the US's insistence that any lasting settlement in Syria requires an end to the Assad regime.
It leaves the US and Russia implacably opposed in their visions for Syria.
"We have regularly and repeatedly expressed our concern about Russian military support for the Assad regime," said John Kirby, the US state department spokesman.
"But we're also watching their actions very carefully.
"If these reports are borne out, it would represent a very serious shift in the trajectory of the Syria conflict and call into question any Russian commitment to a peaceful settlement."
The new US details came in the week that Vladimir Putin gave his strongest admission yet that Russia was already providing some military and logistical support to Syria.
"We are already giving Syria quite serious help with equipment and training soldiers, with our weapons," he said during an economic forum in Vladivostok on Friday, according to the state-owned RIA Novosti news agency.
Until now, Russia's backing has included financial support, intelligence, advisers, weapons and spare parts. Mr Putin insisted it was "premature" to talk of a direct intervention.
However, images emerged last week that appeared to show a Russian fighter jet operating over Syrian soil as well as videos of combat troops speaking in Russian.
Syrian state television showed images of an advanced Russian-built armoured personnel carrier, the BTR-82a, in combat.
Last week the Israeli daily Yedioth Ahronoth cited Western diplomatic sources saying that Russia was on the verge of deploying "thousands" of troops to Syria to establish an airbase from which the Russian air force would fly combat sorties against Islamic State of Iraq and the Levant (Isil).
Those details appear to be backed by satellite images of a Russian base under construction near Latakia, according to anonymous intelligence officials quoted by several American newspapers.
"If they're moving people in to help the Syrian government fight their own fight, that's one thing," one told the Los Angeles Times. "But if they're moving in ground forces and dropping bombs on populated areas, that's an entirely different matter."
Moscow increasingly justifies its support for the Assad regime by pointing to the rise of violent jihadists in Syria with Isil in particular capturing a swathe of the country.
This week, Isil stepped up its programme of cultural cleansing, blowing up temples in the historic city of Palmyra, while fresh clashes along the border with Turkey claimed the lives of 47 fighters at the weekend, according to the Syrian Observatory for Human Rights.
Syria is already home to Russia's only base outside the former Soviet Union - a naval station in Tartus.
The reported build-up of military activity, centred on Latakia and Idlib province, is in areas dominated by the Alawite sect, which counts President Assad among its number.