Secretary of State John Kerry will travel to Kiev, Ukraine, tomorrow to stress U.S. political and economic support after Russian forces' bloodless seizure of Ukraine's Crimea region, a senior U.S. official said yesterday.

Kerry condemned Russia's "incredible act of aggression" in Ukraine and threatened "very serious repercussions."

"You just don't in the 21st century behave in 19th century fashion by invading another country on completely trumped up pretext," Kerry told CBS' "Face the Nation."

"It is really a stunning, willful choice by President (Vladimir) Putin to invade another country. Russia is in violation of the sovereignty of Ukraine," Kerry added.

Kerry said President Barack Obama told Putin in a 90-minute call on Saturday that "there will be serious repercussions if this stands."

Kerry said G8 nations and some other countries are "prepared to go to the hilt to isolate Russia."