Fighting between troops and pro-Russian separatists in Ukraine's southeastern city of Mariupol has left 21 people dead.

Interior Minister Arsen Avakov said 20 insurgents and one police officer were killed.

Mr Avakov said that five policemen were wounded and four rebels were captured.

The Interfax-Ukraine news agency said police were trying to take back control of the police building when they came under fire from pro-Russians who had taken the interior ministry building.

Ukrainian forces brought several armoured cars into the town, a major industrial and shipping centre with a population of about half a million.

Members of an ITV news crew in the city said on Twitter that they saw heavy guns being used and smoke from burning tyres rising above Mariupol.

The fighting occurred as Ukraine commemorated Victory Day celebrations marking the Soviet defeat of German forces in World War II.

Ukraine held low-key ceremonies while Russia, which the West accuses of fomenting Ukraine's insurgency, put on a display of military might in Moscow's Red Square.

President Vladimir Putin has reviewed Russian naval ships in the port of Sevastopol as he visited Russia's naval base in the Crimean port for the first time since the annexation of the Black Sea peninsula.

Ukraine's Foreign Ministry condemned the visit as a deliberate escalation of the crisis.

NATO Secretary General Anders Fogh Rasmussen said that President Putin's visit was inappropriate.

Mr Rasmussen also said he had no visible confirmation of the withdrawal of Russian troops from Ukraine's border.

Russia's Foreign Ministry had suggested that Rasmussen was "blind" after stating he had seen no signs Russia was withdrawing troops from Ukraine's border.