An exchange of mortar fire has been reported in eastern Ukraine, where government troops and pro-Russian separatist forces had been observing an uneasy cease-fire in the past week. The news comes as Secretary of State John Kerry says Russia could face sanctions if it doesn't help end the violence.

Russian state news media are reporting explosions near the airport in the eastern Ukrainian city of Kramatorsk, where the armed groups reportedly absorbed and returned mortar fire. Similar clashes were reported earlier this week.

From Moscow, NPR's Corey Flintoff reports for our Newscast unit:

"Each side in the conflict is accusing the other of violating the cease-fire, which was declared by the Ukrainian government last Friday. On Tuesday, separatists shot down a Ukrainian military helicopter, killing all nine troops on board."The cease-fire is set to expire early Friday, but Ukrainian President [Petro] Poroshenko says he may cancel it earlier if rebels continue to attack. A self-styled rebel leader says he is not expecting any talks with the Ukrainian government anytime soon."

After discussing the Ukrainian situation with U.S. allies in France, Secretary of State John Kerry says Moscow should call on the pro-Russian separatists to disarm within "the next hours."

"Kerry says that additional sanctions against Russia could be discussed when European Union leaders meet on Friday," reports NPR's Jackie Northam from Paris, where she's traveling with the diplomat. "Kerry says it would be preferable not to be in what he calls a sanctions mode."

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