A missile attack from pro-Russia separatists may have killed up to 30 Ukrainian soldiers and border guards Friday in what may be the deadliest attack since the Ukrainian government ended a self-imposed cease-fire late last month.

The rebels fired Grad missiles at a Ukrainian border post in Zelenopillya, in eastern Ukraine near the Russian border, military sources told Reuters.

"Up to thirty (were killed). It is not excluded that the number of victims will rise because these blood-thirsty scum despicably shot from Grad (missile) systems and there is destruction," Zoryan Shkyryak, an adviser to Ukrainian Interior Minister Arseny Avakov, reportedly said. "I think a response will not be slow in coming after this bloody terrorist act.”

After a three-month battle with pro-Moscow rebels, who have set up “people’s republics” in the eastern part of the country and want to join Russia, the Ukrainian army has been making progress against the rebels. Separatists were flushed out of their stronghold of Slaviansk over the weekend, forcing them to hole up in Donetsk, a city of about 900,000 in eastern Ukraine. The Ukrainian government said it has a “nasty surprise” in store for the rebels.

Meanwhile, the heads of France and Germany want Russian President Vladimir Putin, who has much influence in the region, to closely monitor the Ukraine-Russia border, where separatists and their sympathizers are easily transporting weapons and equipment to help the rebels. French President Francois Hollande and German Chancellor Angela Merkel have also called for a renewal of the cease-fire.