U.S. President Barack Obama has expressed skepticism about the effectiveness of a potential no-fly zone over Syria.
In a television interview broadcast late Monday, Obama said if there was a move to restrict flights by the Syrian air force, that "may not be actually solving the problem."
The president also said he will "preserve every option" available to him, and that the U.S. will be involved in a "careful, calibrated way."
The White House has not completely ruled out a no-fly zone as a tool to help bring an end to the Syrian conflict. Syrian ally Russia says it would not permit enforced restrictions of Syrian airspace.
U.S. Ambassador to NATO Ivo Daalder told Reuters that neither the alliance nor the United States is considering a no-fly zone over Syria at this time.
The United States said last week it plans to arm Syrian rebels after uncovering evidence of chemical weapons use.
Meanwhile, Syrian President Bashar al-Assad is warning Europe it will "pay the price" if it arms the rebels fighting to drive him from power.
Assad told Germany's Frankfurter Allgemeine Zeitung newspaper that giving the rebels weapons would "export" terrorists to Europe. He said fighters in Syria would gain combat experience and return to Europe with extremist ideologies.
The Syrian president also denied U.S. accusations that he has used chemical weapons on rebel fighters with deadly results. He called it illogical.