The Ukrainian National Guard briefly drove out pro-Russian militants in the eastern Ukrainian city of Mariupol early Wednesday but later withdrew, making no attempts to retake the administrative center of the city, which has been occupied by separatists for weeks.

According to Ukraine’s Channel 5, Ukrainian forces entered the rebel-held city during the night, smashing furniture and office equipment in the city hall building. Five pro-Russian activists were killed in the clashes overnight and five others were detained, Irina Voropaeva, a spokeswoman for the pro-Russian separatists, said Wednesday.

Ukrainian Interior Minister Arsen Avakov confirmed Wednesday that government troops entered Mariupol and overtook city hall in a joint operation with ministry troops and the army. Avakov added that several people have been detained, including Igor Kakidzyanov, the defense minister of the separatist "Donetsk People's Republic."

The latest clash comes after 30 pro-Russian insurgents and four government soldiers were killed in fighting in the eastern city of Slovyansk on Monday. The airport in nearby Donetsk was shut down for a day and reopened Tuesday.

The Kiev government sent troops to the eastern port city in an effort to restore authority in the region, which has been seized by pro-Russian separatists aiming to have an eastern portion of the country break away from Ukraine and join Russia via a referendum set for May 11.

"They don't want us to hold our referendum, but it's our right. That's democracy," a man identified as Alexander said at the Mariupol town hall on Wednesday. "If this keeps going, we won't even settle for federalization," he added, referring to a proposal for regional autonomy within a federal Ukraine.

Kiev has called for a presidential election on May 25 and has said it is open to new international talks in Geneva. The government has called the proposed referendums set to take place in the regions of Donetsk and Luhansk illegal, since it sees them as a first step for Russia to annex the region, similar to what took place in the Crimean peninsula in March.

NATO Secretary General Anders Fogh Rasmussen said Tuesday the growing tensions in Ukraine are considered “the gravest crisis to European security since the end of the Cold War." He added, "But this is not just about Ukraine. This crisis has serious implications for the security and stability of the Euro-Atlantic area as a whole."