HARRISBURG, Pa.—U.S. Sen. Bob Casey of Pennsylvania is criticizing what he's describing as Russia's "act of aggression" in Ukraine and is calling for a unified response from the United States and its allies.

Casey, a Democrat, said in a Monday statement that Russia's takeover of Crimea "is a blatant violation of Ukrainian sovereignty."

President Barack Obama says the U.S. was considering economic and diplomatic options that will isolate Russia, and called on Congress to work on an aid package for Ukraine.

Casey says the U.S. can support the Ukrainian government with economic assistance, such as loan guarantees. He also suggests he'd support legislation to freeze assets of key Russian officials or impose economic sanctions against Russia.

He also says that if the Ukrainian government asks, civilian observers should be sent.