The United States urges Syria's neighbors to remain "vigilant" of their airspace, a top U.S. official said on Monday, following Turkey's recent grounding of a Syrian airliner over suspicions that it was smuggling arms meant to replenish forces backing beleaguered Syrian President Bashar Assad.

Last week, Turkey forced down a Syrian airliner traveling from Moscow, saying it was carrying Russian munitions destined for Assad's military, infuriating Moscow and Damascus.

Russia has said there were no weapons on the plane and that it was carrying a legal cargo of radar equipment. But it moved to cool friction with Ankara - Foreign Minister Sergei Lavrov said the incident would not hurt "solid" relations.

Later on, on Monday, Turkey ordered an Armenian plane flying to the Syrian city of Aleppo to land and authorities were searching its cargo, as Ankara steps up efforts to prevent its air space from being used to supply the Syrian military.
 
Remarking on recent incidents involving airliners en route to Syria later Monday, U.S. State Department Spokesperson Victoria Nuland warned Syria's neighbors of allowing arms smuggling into the troubled country.

Commenting on Turkey's move to ground the airliner, Nuland said that the U.S. backs "the decision that Turkey has made in light of the apparent violation of their airspace by this aircraft."

"It’s not surprising, frankly, that the Syrians took countermeasures there. We are encouraging all of Syria’s neighbors to be vigilant with regard to how their airspace is used, particularly now that we have this concrete example," the U.S. official added.

Nuland also commented on the possibility that foreign-supplied arms, geared at bolstering Assad's opposition, may be reaching Islamic radicals in the area, saying that, despite the fact that the U.S. insisted that it would provide "only nonlethal assistance," states involved must make sure aid doesn’t fall into the wrong hands.

"We’ve been clear from the beginning that there are issues here as to where this goes and that we need to all work hard to ensure that extremists, jihadists, al-Qaida, other groups who don’t share our larger interest in seeing a democratic Syria emerge from all of this, get their hands on weapons that can be used to exploit the situation, that this truly be an effort on the opposition’s side which reflects a Syria whose future will be democratic, whose future will be inclusive, whose future will reject extremism," Nuland said.

The State Department official added that the U.S. has also been "very clear in calling on all members of the opposition to actively, publicly reject extremists in their midst, reject efforts by extremists to hijack the legitimate aspirations of the Syrian people."