WASHINGTON (AP) — The U.S and allies are trying to rally a Western front strong enough to persuade Russia to step back from a military takeover of Ukraine’s strategic Crimea region. But several major allies acknowledge there are few options beyond already-threatened economic and diplomatic penalties.

On Monday, President Barack Obama described the Russian advance in Crimea as a violation of international law. He called on Congress to approve an aid package for the new Ukrainian government, and repeated earlier threats that the U.S. will take steps to hobble Russia’s economy and isolate it diplomatically, if President Vladimir Putin does not back down.

But French Foreign Minister Laurent Fabius said Russia’s control of Crimea would not be easy to resolve.