U.S. allies "could" arm terrorist groups in Syria if Russia continues to enable President Bashar Assad's attacks on the rebels, according to the State Department, a decision that would make the civil war in that country "much worse."

"That could happen, that's a possible scenario," State Department spokesman Mark Toner told reporters on Monday.

U.S. allies such as Turkey have armed Syrian Islamist groups in the past, in addition to providing "tacit support" to the al Qaeda affiliated al-Nusrah Front operating in the country. Under pressure from the United States, Turkey designated the organization as a terrorist group. But Russian and Syrian attacks on humanitarian aid workers, in the face of U.S. attempts to broker a cease-fire, have raised the possibility that jihadists could once again find favor with some of Assad's enemies.

Toner emphasized that the United States will not arm terrorists, but also said there is no alternative plan to bring a halt to the fighting other than by continuing to ask Russian President Vladimir Putin and the Syrian regime to stop bombing civilians and rebel groups.

"We're committed to continuing to engage with Russia diplomatically and we're not going to walk away from that avenue," Toner said. "Again, the secretary said it would be diplomatic malpractice to do so. As dark as it seems, frankly, it's one of the few options that we have."

When reporters suggested that the State Department has no leverage over Russia, Toner said Putin's current policy could expose his own country to increasingly violent terrorist attacks if the "diplomatic process" breaks down entirely.

"Russia is in a position now where they're supporting the regime and that could expose them to a greater involvement and more of a burden-sharing in order to prop up the regime if the fighting became worse," Toner said.

"I think that those who may be deluded into thinking there is a military solution also have to realize — and we've alluded to this before — that there are those, not the United States, but there are those who back various groups, opposition groups within Syria, who also may seek to arm them and again what you'll have as a result is just an escalation in what is already horrific fighting," he added. "As I said, things can go from bad to much worse."