Noting deadly attacks by Russian-backed separatists who have renewed a push near the southern port city of Mariupol, Ukraine says it can't withdraw heavy weapons from the front lines, as required by a week-old cease-fire.

"Ukraine's military says two government soldiers were killed and about 10 wounded in the past 24 hours," NPR's Corey Flintoff reports from Moscow. "A government spokesman say its positions were hit by shelling 27 times in the past day. Meanwhile, of course, the separatists are claiming that the government initiates the attacks."

The observance of the cease-fire that took effect in Ukraine last Sunday remains a subject of debate. Days after it began, separatists fighters surrounded the town of Debaltseve, a railroad hub. Now Ukraine worries that in Mariupol, another strategic area is being targeted.

Despite the cease-fire, "shelling from the Russian side has continued along the Mariupol front," the Kyiv Post reports, "and Ukraine's military expect the Azov Sea port city of 500,000 people will be the target of a major Russian assault within the next weeks."

In some areas, the clash has quieted down. Over the weekend, the two sides performed a prisoner exchange; another is tentatively planned for March. But even as they urge an end to the fighting, the U.S. and its European allies say new sanctions could be announced against Russia if the cease-fire fails.

"Tensions are high in Ukraine, where tens of thousands of people rallied in Kyiv over the weekend," Corey reports. "They were commemorating the deaths of scores of people who were killed by sniper fire at demonstrations a year ago, and the ouster Ukraine's former president."

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