Ukrainian forces battled pro-Russian militants in Eastern Ukraine this weekend as militants tried to recapture buildings in Odessa in a conflict that killed more than 40 people.

According to USA Today, Ukrainian troops also moved against pro-Russian rebels in Slovyansk, a city in Eastern Ukraine that has been marred by clashes between the Ukrainian military and pro-Russian separatists.

The United States and Kiev believe that the rebels are trying to overthrow Ukraine's elected government with the help of Moscow.

Ukrainian Interior Minister Arsen Avakov said that around 800 pro-Russian forces are shooting large-caliber weapons and mortars. He added that four officers were killed, and 30 were wounded in violent clashes.

In the Black Sea port city of Odessa, pro-Russian rebels vowed to recapture buildings after attending the funeral of a politician that died in a fire that killed more than 40 people.

Vyacheslav Markin, a regional parliament member and opponent of the Ukraine government, was buried on Monday to the sounds of hundreds of pro-Russian supporters shouting "Hero, hero!"

Markin died Sunday after he and 41 others were caught in a fire that erupted due to violence Friday between Ukrainians who want independence from Kiev's government, and Ukrainians who support Kiev and believe that Markin wanted to secede to Russia.

"This city has never seen such upheaval, such moments in history are not forgotten," said Pyotr Volkov, a doctor and a former lawmaker who was a friend of Markin's.

Ukrainian Prime Minister Arseniy Yatsenyuk visited Odessa, and said that Russia is directing "well-prepared and organized action against people, against Ukraine, and against Odessa."

Odessa's conflict worsened Friday when supporters of Ukraine's government tried to get rid of an encampment of pro-Russian sympathizers in Kulikovo Field Square. Ukraine-supporters burned their tents, so pro-Russian supporters went to the trade union hall. Gas bombs were thrown, which are believed to have ignited the fire that killed more than 40 people.

"I am shocked that this happened, I can't believe it," said Svetlana Lvova, a teacher who came to lay flowers in the trade union hall, which is now nothing but charred remnants. "Husbands and wives are fighting with each other about what side to take."

Flowers and candy were left at the site of the devastation, which is scattered with bloody clothes and debris.

Pro-Russians also strong-armed the police headquarters in Odessa Sunday and won the release of 67 individuals who were detained due to fighting.

Kiev blames the pro-Russian separatists for beginning the violence, while ethnic Russians blame pro-Ukraine supporters.

"I don't want to be called a separatist. We don't want to secede from Ukraine," said Anatoly Kuznetsov, an archivist who lives in Odessa but whose family has Russian roots. "But we hope that Russia won't leave us in our sorrow. We are weak. We are not aggressive."

Moscow is currently threatening to invade Ukraine to protect the Russian-speaking population.