WASHINGTON (AP) - The White House says the United States is concerned about new signs of violence and disturbances by rebels in Ukraine.
Ukraine's acting president says 12 troops died when rebels shot down a military helicopter in Slovyansk using a portable air defense missile.
White House spokesman Jay Carney says the U.S. hasn't verified what happened. But he says the U.S. is concerned it indicates separatists still have access to advanced weapons and are getting help from outside Ukraine. He's alluding to Russian support.
Carney also says it's unacceptable that insurgents have detained four observers from the Organization for Security and Cooperation in Europe. He says they should be released immediately.
Carney is calling on Moscow to use its influence to get rebels to disarm and participate in Ukraine's political process.