DONETSK, Ukraine — Four Ukrainian soldiers were killed and 30 more were wounded in gun battles in eastern Ukraine on Monday after pro-Russian rebels ambushed their squad and shot down a military helicopter.

Ukrainian forces in Sloviansk clashed with up to 800 heavily armed pro-Russian separatists around 10:45 a.m. local time, Interior Minister Arsen Avakov told the Interfax news agency.

The separatists opened fire on Ukrainian forces from behind unarmed civilians, who were used as human shields, and set fire to cars and nearby buildings, according to a statement from Ukraine's Interior Ministry.

“In the morning, a squad in the anti-terrorist operation was hit by an ambush by terrorist groups. They are using heavy weapons,” Avakov said.

Around 2:45 p.m., pro-Russian insurgents used a heavy machine gun against a Ukrainian M-24 helicopter in Sloviansk, downing it in a nearby river. The chopper's pilots managed to escape without injury.

Both pro-Russian separatists and local civilians were among those killed during the fierce morning confrontation that unfolded near a checkpoint on the city’s southeast border recaptured by Ukrainian forces and visited by Mashable over the weekend. An army captain said his men had rolled in on armored personnel vehicles and engaged armed rebels guarding the checkpoint. They set it ablaze in an attempt to deter the Ukrainian soldiers, but they soon retreated after taking fire. On Sunday, the checkpoint smoldered for several hours.

During the clashes on Monday, separatists fired at a minibus carrying those wounded in the battle, killing one Ukrainian officer in an elite police unit who was escorting the minibus, according to the Interior Ministry.

On the separatists’ side, up to 20 of their fighters were reportedly killed during the Ukrainian forces’ assault, one rebel told Interfax. Mashable was not able to confirm this.

“We have many dead. Maybe more than 20 people,” the rebel representative said, adding that the militias managed to stop government forces from advancing further into the city after “great trouble.” The rebels earlier said between 12 and 15 of their fighters had been injured.

Russian state news agency RIA Novosti quoted another rebel leader, Igor Strelkov, who said his side "lost around 10, including peaceful residents, and 20-25 were wounded."

A bloody weekend

A man lays flowers inside the burnt trade union building in Odessa, Ukraine, Sunday, May 4, 2014. More than 40 people died in the riots, which some from gunshot wounds, but most in a horrific fire that tore through the trade union building late Friday. Image: Sergei Poliakov/Associated Press

Since last Friday, Ukraine has vigorously moved to quell the armed rebellion of Moscow-supported separatist fighters who have laid siege to the region. Previous attempts ended in failure when several soldiers defected to the pro-Russian rebel side. Rebel forces seized several armored military vehicles and dozens of weapons from soldiers who turned them over after being stopped by civilians who blocked them from entering the cities of Sloviansk and Kramatorsk in March. In the latest iteration of the operation, Ukrainian forces managed to retake a television tower in nearby Andriivka and encircle Sloviansk.

During the Ukrainian anti-terrorist operation carried out over the weekend, members from the elite “Alpha” unit of the Security Service of Ukraine (SBU) neutralized a sniper nest in a television tower in Kramatorsk, about three miles south of Sloviansk, the Kyiv Post quoted a Marina Ostapenko, SBU spokesperson, as saying during a briefing in Kiev on Monday.

Ostapenko said two Ukrainian soldiers were wounded in the operation, but that it had been successful. She told reporters that sniper nests in other parts of the embattled region had been discovered, but not yet cleared by SBU officers.

The security service also seized a 3.3-pound “potentially radioactive” substance in Ukraine's western Chernivtsi region. Ostapenko said the SBU suspects that pro-Russian separatists may have been planning to detonate a “dirty bomb” there. Ten suspects were arrested in connection to the substance, including a Russian citizen.

Pro-Russian separatists have seized more than 20 key government buildings in at least 11 cities and towns in Ukraine’s gritty industrial eastern region, where a majority of the population speaks Russian as their first language, and pro-Russian sentiment is high, given its proximity to and history with its eastern neighbor.

Russian President Vladimir Putin has said that Russia reserves the right to intervene in Ukraine, much like it did in Crimea. The Russian government said it annexed the Black Sea peninsula in March in order to protect ethnic Russians and Russian speakers from the new Kiev government, which it calls "neo-Nazi and fascists."

Pro-Russian protesters celebrate on top of a truck at a police station in Odessa, Ukraine, Sunday, May 4, 2014. Several prisoners that were detained during clashes that erupted Friday between pro-Russians and government supporters in the key port on the Black Sea coast were released under the pressure of protesters that broke into a local police station and received a hero's welcome by crowds. Image: Vadim Ghirda/Associated Press

On the heels of Friday's tragic government building fire in the southern port city of Odessa that killed dozens of pro-Russia demonstrators, the fresh clashes on Monday threaten to further escalate tensions between Ukraine and Russia, which has massed tens of thousands of troops on its border, within striking distance of its former Soviet subject.