US intelligence is saying that Russia is constructing a large air base near the port city of Latakia to support President Bashar Assad. If true, the base would represent a substantial escalation of Russia's role in the conflict and put Russian forces on a collision course with American-backed rebels.

Telegraph:

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 rights to fly over neighbouring countries with military cargo aircraft during September, 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.

Mr Obama on Friday met King Salman of Saudi Arabia to repeat their demand that any lasting settlement in Syria would require an end to the Assad regime.

It leaves the US and Russia implacably opposed in their visions for Syria.

John Kerry, Secretary of State, telephoned his Russian counterpart to express US concerns on Saturday.

"The secretary made clear that if such reports 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," the department said.

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.

In fact, Russia has been propping up the Assad regime for years, keeping the economy afloat, replacing armor lost in the war, supplying intelligence, and training its soldiers. Assad's regime would have collapsed years ago without Russia's assistance. 

Until now, Vladimir Putin has been content to keep the pot boiling in Syria - not giving Assad enough to win and providing just enough aid to prevent him from losing. But it appears that Islamic State forces are now making significant progress, demanding that Russia itself enter the fray:

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 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."

Russia may or may not be able to differentiate between US backed rebel forces and ISIS fighters. It may not even be important to them. If they bomb the rebels, what's President Obama going to do? Go to war? More likely, he will dispatch the "strongly worded letter" to chide President Putin. 

Meanwhile, The Russians appear to be ready to contribute to the chaos and bloodshed in Syria that has already cost the lives of 250,000 and created 11 million displaced people - half the population of Syria.