US Secretary of State John Kerry has threatened Russia with tougher economic sanctions if it fails to back down from its chaotic involvement in Ukraine.Kerry told the Senate Foreign Relations Committee in Washington that what the US sees from Russia is an illegal and illegitimate effort to destabilize a sovereign state and create a contrived crisis with paid operatives across an international boundary.President Barack Obama and Congress have imposed sanctions on Russia in response to the annexation, and Kerry signaled the additional penalties on Russian energy, banking and mining could be imposed if Moscow fails to honor a sovereign Ukraine.Kerry said he would meet next week in Europe with Russian Foreign Minister Sergey Lavrov and Ukrainian officials. He said Russia has a choice and can work with the international community.