Government troops in eastern Ukraine have won back the port city of Mariupol from pro-Russian separatist rebels after heavy fighting.
Interior Minister Arseny Avakov said the security forces had successfully brought separatist strongholds in the city "under control".
Five pro-Russian rebels were reported killed and at least four government soldiers injured in the city. Rebels elsewhere in the region have confirmed they now have three tanks.
The appearance of the tanks, filmed in various towns in the Donetsk region, sparked a row between Kiev and Moscow, with Ukrainian President Petro Poroshenko protesting to his Russian counterpart, Vladimir Putin.
Russia denied Ukrainian allegations that the tanks had entered Ukraine from its territory.
Hundreds of combatants and civilians have been killed since pro-Russian separatists in the Donetsk and neighbouring Luhansk regions declared independence after holding referendums last month, which were deemed illegal by the government in Kiev.
Friday's operation seems to have been a success after many setbacks for the Ukrainian forces, the BBC's David Stern reports from the Ukrainian capital, Kiev. However, Mariupol has been fought over intensively before and it cannot be ruled out that the insurgents may return to try to take it back again, our correspondent adds.
'Flag raised' Avakov said government forces had raised the national flag over Mariupol's council building.
The port city lies on the Sea of Azov in the south of Donetsk region which has changed hands several time during the conflict.
The operation had begun at 04:50 (01:50 GMT), the interior minister added. Confirming that four soldiers were injured, Ukraine's National Guard said on its website that its recently formed Dnepr and Azov "volunteer" units were being used in the operation. A rebel source told Russia's Interfax news agency that five fighters had been killed.
Several charred rebel armoured cars could be seen in the streets after the battle.
Video of a ferocious gun battle, which appears to show government soldiers at a barricade in Mariupol, was posted on YouTube. In another development, a bomb destroyed a minibus in the city of Donetsk on Thursday evening, killing two people and injuring two.
The vehicle belonged to Denis Pushilin, head of the self-styled Donetsk People's Republic, who was not using it at the time.
'Some hope' Pushilin confirmed the rebels had received the three tanks and that they were in Donetsk, without saying where they had come from.
The tanks, he told the Russia 24 news channel, gave the rebels "at least some hope of defending [Donetsk] because heavy weapons are already being used against us".
A paramilitary in the rebel-held town of Snizhne, where the tanks were first sighted, told Reuters news agency: "We got them from a military warehouse."
Contrary to initial reports that the tanks are Russian-made T-72s, one of the most commonly used models in late Soviet times, rebels and bloggers have identified them as T-64s, an older model manufactured in Ukraine.
One amateur video shows the tanks passing through the town of Torez.
Avakov has said the tanks crossed the border from Russia along with armoured personnel carriers and artillery pieces in the Dyakove area of Luhansk region, before moving into the neighbouring Donetsk region.
Ukraine and the West accuse Russia of supporting and arming the rebels - a claim denied by Moscow.
The rebellion began after February's ousting of the elected President, Viktor Yanukovych, whose last-minute decision not to sign a landmark treaty with the EU in November sparked mass protests in Kiev.