Russia has sent artillery units and seven tanks to a Syrian air base as part of Moscow's continued military buildup in the war-ravaged nation, a US official said Monday.

The increase of Russian hardware in Syria has caused concerns in the West about the implications of Moscow militarily helping its old ally, President Bashar al-Assad.

Speaking on condition of anonymity, the US official told AFP the seven T-90 tanks arrived in recent days but had not been sent outside the airbase near Latakia, on Syria's Mediterranean coast.

And the artillery, which arrived last week, appears to have been sent there to protect the facility. There was no indication Russia had sent fighter jets or helicopter gunships to Syria.

"It appears, and all the indications are pointing, that (the artillery is) for airfield defense," the official said.

Another US official, also speaking on condition of anonymity, said Russia has installed enough modular housing units to house about 1,500 people.

It is not easy to gauge how many Russian troops are already on the ground, but the official said the number is in the "hundreds."

Russian help for Assad could seriously complicate the US-led coalition's air strikes against Islamic State jihadists in Syria, with defense officials worried about the possibility of accidents if coalition and Russian planes operate in the same airspace.

Two Russian transport planes, purportedly carrying humanitarian aid, landed in Syria on Saturday, Russian state media said, and Russian Foreign Minister Sergei Lavrov last week said Moscow was sending military equipment along with aid "in accordance with current contracts."

At a news briefing, Pentagon spokesman Captain Jeff Davis said Russia apparently was establishing a forward air operations base in Latakia.

"It's been a continued steady flow (of equipment) for the last week and a half or so," he said.

AFP reported last week that dozens of Russian naval infantry had arrived in Syria, along with two tank-landing ships and about a dozen Russian armored personnel carriers.

Russia is a staunch ally of the regime in Damascus and maintains a naval facility in Tartus province.

"We would welcome Russian contributions to the overall global effort against (the IS group) but things that continue to support the Assad regime, particularly military things, are unhelpful and risk adding greater instability to an already unstable situation," Davis said.

More than 240,000 people have been killed in Syria since the conflict began with anti-government protests in March 2011.

The government has lost large swathes of territory to rebels and jihadists such as the Islamic State group.

Hollande says air strikes against IS in Syria 'necessary'
Paris (AFP) Sept 14, 2015 - French President Francois Hollande, whose country is carrying out surveillance flights over Syria, said Monday it would be "necessary" to carry out air strikes against Islamic State militants there.

"We announced surveillance flights that would allow us to prepare air strikes if they were necessary, and they will be necessary in Syria," he told journalists after talks with Nigerian President Muhammadu Buhari.

France has been part of a US-led coalition bombing IS positions in Iraq for the past year, but Hollande changed strategy last week to expand action to Syria as political pressure mounts over the jihadist threat faced by France.

Hollande said last week he had ordered surveillance flights because he wanted to find out "what is being prepared against us and what is being done against the Syrian population".

Hollande's move to carry out air strikes in Syria also comes as Europe is buckling under the pressure of a flood of refugees from the war-torn country.

France has so far confirmed it has carried out two surveillance flights over Syrian territory.

The United States, Canada, Turkey and Gulf states have already been involved in strikes on IS militants in Syria.

Australia also announced last week it would join the Syrian operation.

Britain killed two jihadists in a drone attack in Syria last week.