The U.S. military is preparing to provide protection and support for the American-trained Syrian forces who will fight in that nation's four-year-old civil war, top officials say.

But that prospect is reviving concerns about Syrian air defense systems and the risk that U.S. aircraft face when flying bombing missions over the war-torn country.

Top military officials say there is no doubt that some level of U.S. support — most likely close-air support and intelligence — will be needed to support the moderate Syrian rebel force that may emerge from an American-led training effort later this year.

"The forces that we train in Syria, we will have some obligation to support them after they're trained," Defense Secretary Ash Carter said at a press briefing Wednesday. "That is something we are aware of and discussing."

The U.S. military has screened at least 1,200 Syrian rebels who could participate in a new training program and eventually fight against Islamic State extremists, also known as ISIS or ISIL, on the Syrian side of the border. The goal is to select and train about 5,000 fighters over the next year at facilities in Turkey, Saudi Arabia and Qatar.

"The program won't succeed unless they believe themselves to have a reasonable chance of survival," Army Gen. Martin Dempsey, chairman of the Joint Chiefs, said on Capitol Hill on Wednesday.

"It was always my advice that we had to come to some conclusion to assure them that they would be protected. Now, the scope and scale of that protection is the part of this that's being actively debated," Dempsey told lawmakers.

Yet supporting those fighters could be far riskier than the current operations in Iraq because the U.S. has an adversarial relationship with the embattled regime of Syrian President Bashar Al-Assad. And Assad controls an intimidating arsenal of surface-to-air missiles, including medium-range SA-17s.

To date, Assad has not used his air-defense weapons against U.S. aircraft, which began a bombing campaign last year and continue to make daily flights into Syrian air space.

U.S. military officials say the Syrian air defense system so far has remained "passive," part of an implicit agreement that U.S. aircraft limit their targets to Islamic State forces, which the Syrian regime is also seeking to defeat.

However, American-trained moderate Syrian rebels would be fighting both Islamic State forces as well as the Syrian regime. Providing U.S. support for those rebels could put the American military in direct contact with Syrian forces for the first time.

Top military officials are wrestling with the question of how to respond if and when the American-backed rebels confront the Syrian regime.

"We are still working through how we would react to that eventuality," Carter said Wednesday.

"It is a situation that we have foreseen and it's not a situation where decisions have been made. It would depend very much on the situation or the circumstances," Carter said.

Many experts say the Syrians could activate their surface-to-air missile systems and immediately put U.S. fighters, bombers and manned and unmanned surveillance aircraft at risk.

That, in turn, would require the U.S. operations over Syria to become far more complex. More stealth and electronic attack capabilities would be needed. It could potentially prompt a round of air strikes on the Syrian military facilities that house the missile systems, which would significantly escalate U.S. involvement in the conflict.

Some experts say Syrian air-defense systems have deteriorated over the past few years, with the civil war limiting the Syrian military's ability to maintain them in good working order. In addition, Islamic State forces have seized several key facilities.

"My sense is that [the Syrian military's] capability has declined, from loss of territory, lack of activity, lack of training," said Jeffrey White, a defense fellow at the Washington Institute for Near East Policy who has studied the Syrian air-defense system.

"The great unknown what the Russians and others may be doing in order to maintain at least part of it," White said.

He said U.S. or coalition air forces would be able to deal with Syria's air defense system "without too much difficulty, but that doesn't mean it would be no risk. You can't say it would not be a factor. I would imagine that the military planners would devote quite a bit of effort to make sure we could deal with this."