Washington (AFP) - Russia's decision to deploy its most hi-tech air defense system to its base in Syria is raising "significant concerns" for the US military, a US official said Wednesday.

Moscow says it is sending S-400 anti-aircraft missiles to Latakia in northwestern Syria, in a move that comes after Turkey shot down a Russian fighter jet in the increasingly crowded air space along the border on Tuesday.

The S-400 missiles have a range of about 400 kilometers (250 miles) -- meaning they could reach deep into Turkey or pose a potential threat to US-led coalition planes -- adding yet another dangerous element to an already volatile mix of competing military interests in Syria.

"It's a capable weapons system that poses a significant threat to anyone," a US official speaking on condition of anonymity told AFP. "There are significant concerns related to air operations in Syria."

The United States has since August 2014 led a coalition that has flown more than 8,000 bombing runs against Islamic State group targets in Syria and Iraq.

Russia, too, is dropping bombs in Syria, but these are mainly in different parts of the country from where US and coalition planes are flying. The West says Russia is propping up the regime of President Bashar al-Assad, and not focusing on IS jihadists.

Though Russia and the US-led coalition have agreed on a set of guidelines aimed at ensuring pilots stay out of each other's way, the prospect of batteries of Russian anti-aircraft missiles arriving in Syria is nonetheless raising eyebrows in the Pentagon.

But another US official, also speaking anonymously, said the S-400s "shouldn't" affect coalition flights.

"We are not going to interfere with (the Russians') operations and they are not going to interfere with ours. There's no reason for us to be targeting each other," the official said.

He also noted that Russia in the past week has delivered more than 30 T-90 and T-72 tanks to Latakia.

It was not clear if these were for use by the Russian military or will be provided to forces loyal to Assad.

Story continues

- 'Russian strike' on aid trucks -

Russia resumed its bombing campaign Wednesday and continued to operate close to the Turkish border.

In northern Syria's Aleppo province, apparent Russian strikes hit the town of Azaz and the border area around the Bab al-Salama crossing, the Syrian Observatory for Human Rights monitoring group said.

The monitor and local Syrian activist Maamun al-Khatieb reported three people killed in the strikes, which also set fire to several trucks parked in a lot not far from the crossing.

"Three people have been killed and six injured, most of them are truck drivers," Khatieb told AFP.

He said the trucks were carrying aid and goods for sale, and were parked in a lot where vehicles gather after crossing the border, around three kilometres away.

A US official said he was "aware of reports that a convoy of humanitarian relief vehicles was hit by an air strike today in the vicinity of Azaz."

The official noted that no coalition planes had been near the area for the last 24 hours.

"We are continuing to monitor the situation to determine all of the facts behind the incident," he said.

On Tuesday, Turkey shot down a Russian aircraft along the Syrian border, and rebels killed one of the pilots as he parachuted down after ejecting from the plane. A second pilot was rescued by Russian and Syrian special forces.

A Russian rescue helicopter was also destroyed by rebels, who apparently used a US-made TOW missile.

The prospect that Syrian rebels used US weaponry to kill a Russian further raises concerns that the Syria conflict could devolve into a proxy war.