The Pentagon acknowledged Monday that the Russian military is building a forward air base in Syria, a move that could complicate U.S. and coalition effort to mount daily airstrikes against Islamic State militants.

"We have seen indications in recent days that Russia has moved people and things into the area around Latakia and the air base there that suggest it intends to establish some sort of forward air operating base," Navy Capt. Jeff Davis, a Pentagon spokesman, told reporters.

The provocative Russian military activity in Syria began in early September, Davis said.

Satellite imagery suggests the activity includes reinforcing aircraft runways, building helicopter pads, installing mobile housing units, and constructing several new buildings including an air traffic control tower, according to Stratfor, a private intelligence company.

On Monday, Russia was positioning tanks and artillery along the perimeter of the air base, according to a Reuters report.

The Russian activities have "been progressing on a daily basis," Davis said.

The buildup is one of Moscow's most aggressive military provocations in years, with the apparent goal of boosting the military defenses of its ally, embattled Syrian President Bashar Assad, whose forces have suffered a series of battlefield losses to Islamic State extremists and other rebel groups.

But the move poses risks as U.S. aircraft fly dozens of daily sorties over Syria and launch airstrikes against Islamic State targets on the ground.

Latakia is about 100 miles from Islamic State-held territory near Aleppo.

"Clearly, one of the concerns we have is de-confliction. So I think it's going to be important, as this manifests itself, whatever this is, that we address the issue of de-confliction," Davis said.

Davis declined to comment on media reports Friday that Russia plans to send some advanced anti-aircraft weaponry, specifically SA-22 surface-to-air missiles, to Syria.

For now, Davis said, there is no increased threat to the U.S. pilots flying over Syria and other U.S. personnel deployed in the region.

"We monitor the threat very carefully and we ensure that, where we have coalition aircraft and service men and women operating, that we are able to do that in a safe way," he said.

Davis declined to say how many Russian troops might be in Syria. He also declined to provide details about what the Russians may be shipping to Syria, but said unequivocally that U.S. intelligence has seen no evidence that the Russians are bringing in fighter aircraft or helicopter gunships.

Despite the concerns about de-confliction, the U.S and Russian militaries have had no direct contact lately, Davis said. The relationship between the two longtime Cold War adversaries is severely strained and communications is limited to diplomatic channels

Latakia is a political stronghold for Assad and his Shia Alawite sect. It is widely believed that Assad would return to the Latakia area if the government's capital in Damascus fell to rebels or Islamic extremists.

Latakia is about 60 miles north of Tartus, a port city where the Russians maintain a small naval base.

Russia reportedly also has used Iraqi airspace to fly large transport aircraft into Syria. U.S. officials have urged allies in the region to deny Russia the use of their airspace. But despite having about 3,500 U.S. troops on the ground in Iraq, the U.S. has no direct control over Iraqi airspace.

"It's their sovereign air space and we're there at their invitation and coordinating with them," Davis said.

In late August, Alligator-class and Ropucha-class Russian landing ships were sighted moving south through Turkey's Bosphorous Strait and heading toward the Syrian coast. Photographs appeared to show armored ground vehicles on board the vessels.