Pro-Russian militia forces blockaded Ukrainian region of Crimea's two main airports on Friday.

Arsen Avakov, the Interior Minister of Ukraine, confirmed the news, calling the foreign presence "an armed invasion" and "violation of all international agreements and norms", the BBC has reported.

The airport in Crimea capital Simferopol was the first to be occupied, as armed men, some with Russian flags, arrived overnight. Some of them described themselves as "volunteers" from "the People's Militia of Crimea", a structure that was formed this week amidst the first sparks of tensions between Crimea Tatars and ethnic Russians in the region.

Later the occupation of the Simferopol airport was lifted. 

A spokesman for the Simferopol airport in Ukraine's Autonomous Republic of Crimea, Ihor Stratilati, told the Ekho Moskvy radio station early on Friday morning that the armed persons who had raided the airport were already outside of its premises.

Sevastopol military airport was also seized by men patrolling with armoured vehicles and claiming their aim was to "maintain order". 

Interior Minister Avakov reported that Ukrainian military and border guards have remained inside the facility, which is located near the Sevastopol-based Russian Black Sea Fleet.

Tensions have heightened throughout Crimea between the ethnic Russian majority and Crimea Tatars in the past few days over the prospects of a secession on the peninsular region, where the population is opposed to the new leadership in Ukrainian capital Kiev.

On Thursday a referendum was announced and scheduled for May 25 by the Crimean Parliament on expanding the region's autonomy.

Hours earlier, both parliament and government headquarters in the capital Simferopol were seized by Russian man, and Russian flags were raised over both buildings.

On Thursday and Wednesday, Simferopol also saw scuffles during rival rallies of Russians and Tatars siding with Ukrainians against possible administrative changes.

Kiev's government, which was approved by Parliament on Thursday, has pledged to keep Ukrainian territorial integrity and warned Russia against interference in its own domestic affairs.

Moscow, though assuring it will not breach the sovereignty of Ukraine, put on Thursday 150 000 combat troops on "high alert" at its Ukrainian border and has expressed readiness to react in case of violence on the peninsula.