The U.S. military veered toward declaration of a no-fly zone in Syria Monday, but stopped short of using the controversial term that would set the stage for a direct confrontation with the Syrian air force. 

"It's not a no fly zone," Peter Cook, a Defense Department spokesman, told reporters Monday. 

"But," he added, "the Syrian regime would be wise to avoid areas where coalition forces have been operating. And we will continue defend them and, if need be, we will send aircraft again to defend our forces."

The comments came after Syrian government warplanes on Aug. 18 launched airstrikes on an area near a team of U.S. special operations troops who were providing combat support for Syrian Kurdish rebels fighting the Islamic State group near the town of Hasakah. Hasakah is a key transit point between Mosul and Raqqa, the two major cities of the Islamic State group's territory.

On the following day, two U.S. F-22 aircraft intercepted Syrian Su-24 combat jets flying through the same area, defense officials said.

It was the first time that Syrian aircraft appeared to threaten U.S. troops since the Pentagon began deploying American special operations forces into Syria last year. There are currently about 300 American troops in Syria. 

Cook declined to say whether U.S. aircraft are conducting routine combat air patrols in the airspace near U.S. forces.

"We are going to tell the Syrians and anyone else who may threaten our force in that area that we will defend them and they have a right to defend themselves as well," Cook said.

He suggested that the same warning will apply to other areas where U.S. forces may move in the future.

"We will continue to defend our forces in our fight against ISIL … and as our forces move in Syria and continue their partnered operations we will do what we need to do to protect our forces."

Cook stopped short of threatening to shoot down a Syrian or Russian plane that might threaten U.S. forces, saying: "We’re not going to get into our rules of engagement."

The U.S. and Syria have no formal military-to-military relationship.

Cook said the U.S. relayed the warning to the Syrian regime through the Russian military.

The U.S. and Russia have a "memorandum of understanding" that creates lines of communications for safely sharing the skies over Syria and avoiding a mishap that could spark a broader conflict.

"We did engage through our memorandum of understanding with the Russians, specifically after that instance, to have them communicate to the Syrian regime our concerns about what had happened and the fact that it shouldn’t happen again," Cook said Monday.

For years U.S. military officials have expressed concerns about a confrontation with the Syrian air force and more specifically the Syrian regime's potentially formidable anti-aircraft defenses.

In 2013, Gen. Martin Dempsey, then chairman of the Joint Chiefs of Staff, told Congress that imposing a no-fly zone over Syria would cost as much as $1 billion a month, require thousands of U.S. troops and put U.S. aircraft at risk of being shot down.