Creating a humanitarian safe zone in Syria would entail a "major combat mission" requiring US troops to fight Islamist jihadists and the Damascus regime, Defense Secretary Ashton Carter told lawmakers on Wednesday.
Turkey has long called for a safe area to be set up along the Syrian-Turkish border to protect civilians but President Barack Obama's administration has yet to endorse the idea.
Carter emphasized the challenges involved in establishing a buffer zone, and warned that other regional governments might not be ready to contribute to the effort.
"We would need to fight to create such a space and then fight to keep such a space and that's why it's a difficult thing to contemplate," Carter told members of the Senate Appropriations defense subcommittee.
Carter was asked about the option by Senator Dick Durbin, one of four senators to send a recent letter to Obama calling for setting up a safe area in Syria.
"Though this may not be a genocide by classic legal definition, it is the humanitarian crisis of our time ... with no end in sight," Durbin said.
Carter said such a safe zone would be "contested" by the Islamic State group and other extremists on the one hand and the Syrian regime's forces on the other.
The "practicalities" would be "significant," he said.
The US military's top-ranking officer, General Martin Dempsey, said American commanders have drawn up contingency plans for a safe zone in consultation with their Turkish counterparts.
"We've been planning for such a contingency for some time," Dempsey told the same hearing.
He said American forces were capable of carving out a buffer zone in Syria but it was a major political decision and would mean troops stationed elsewhere would not be available for other missions.
"It's practical militarily but it would be a significant policy decision to do so," Dempsey said.
He also said that for "this to be practical and effective, it would have to involve regional partners."