The U.S. government is intensifying its intelligence sharing and military consultations with Turkey behind the scenes as both countries confront the possibility that Syria’s civil conflict could escalate into a regional war, according to U.S. and NATO officials.

The Obama administration has said it wants to avoid getting drawn militarily into Syria and for months has resisted pressure from Arab allies and some Republicans to back Syria’s rebel groups more forcefully.

But as Syria’s internal conflict has increasingly spilled across its northern border into Turkey, the U.S. government has stepped up cooperation with its key NATO ally. In recent weeks, military officials from both countries have met to make contingency plans to impose no-fly zones over Syrian territory or seize Syria’s stockpiles of chemical and biological weapons, U.S. officials said.

U.S. intelligence agencies were also the source of a tip that led the Turkish military to intercept and ground a Syrian passenger plane en route from Moscow to Damascus last week on suspicions that it was carrying Russian-made military hardware, according to U.S. officials.

The Syrian plane was carrying “radar and electrical parts for Syria’s Russian-made antiaircraft systems,” one U.S. official said, speaking on the condition of anonymity to discuss details of the sensitive operation. Syria has relied on Russia for decades to help build its radar and antiaircraft defenses, among the most extensive in the Middle East.

The plane grounding sparked a diplomatic dust-up among Turkey, Russia and Syria and further exacerbated tensions that erupted Oct. 3 when Syria fired shells across the border and killed five Turkish civilians.

Since then, cross-border shelling has continued as the Syrian military has attacked rebel groups along the frontier, with rounds sometimes landing in Turkish territory. Turkey has retaliated with artillery strikes, most recently on Friday, while warning Damascus that the risk of all-out war is increasing.

The United States and NATO have publicly supported Turkey, saying it has a right to act in self-defense. At the same time, they have called for restraint and repeated that neither Washington nor Brussels has any intention of getting involved militarily.

Behind the scenes, however, the border clashes have changed the strategic calculus and led U.S. military and intelligence officials in particular to collaborate more closely with Turkey.

“I can certainly assure you that our militaries, our military officers, are in contact,” Francis J. Ricciardone Jr. , the U.S. ambassador to Turkey, told journalists in Ankara on Tuesday. “This week I know there is a special focus of our military experts talking about Syria. And what militaries do well is plan for every contingency and every eventuality.”

Ricciardone said “no political decision has been made” regarding whether to support or impose a no-fly zone in Syrian territory to protect civilians or opponents of the government of Bashar al-
Assad in Damascus, but he acknowledged that U.S., Turkish and NATO officials were discussing options.