Ukrainian Interior Minister Arsen Avakov said on Friday that Russian naval forces had taken control of the military airport in the Black Sea port of Sevastopol in Ukraine’s mainly-Russian-speaking Crimea region.

"I consider what has happened to be an armed invasion and occupation in violation of all international agreements and norms," Avakov said in a statement posted on his Facebook page.

One of Russia’s naval fleets is based at Sevastopol.

Separately, dozens of unidentified armed men dressed in military uniforms were seen patrolling the airport at Simferopol, Crimea’s capital early on Friday morning.

The men, who refused to talk to journalists, did not move into the passenger terminal and appeared to be allowing the airport to function normally, the RIA-Novosti news agency quoted a worker at the airport as saying. Their uniforms were without military insignias and it was not immediately clear who they were.

The situation at the airports has added to concerns of growing separatism on the Black Sea peninsula following the ouster of Kremlin ally President Viktor Yanukovych last week.

On Friday, members of the regional parliament voted to hold a referendum on May 25 on increasing the region's autonomy from Kyiv.

Cross-border tensions

The events in the Crimea region have heightened tensions with neighboring Russia.

Ukraine's parliament adopted an appeal Friday for Russia to "stop moves that show signs of undermining national sovereignty and territorial integrity ... (and) reject support for separatism in Ukraine, of any form."

Ukraine's interim leader Oleksandr Turchynov also told parliament that he was summoning the country's security and defence chiefs over the crisis in Crimea.

On Wednesday, President Vladimir Putin ordered an urgent drill to test the combat readiness of his country's armed forces in the west of the country.

Russia has also granted shelter to Ukraine's ousted president, Viktor Yanukovych, who is expected to hold a press conference later on Friday.

Yanukovych said in a statement released on Thursday that he still considered himself Ukraine's legitimate leader despite being voted out of office by parliament last week.

Switzerland, Austria take action

Switzerland and Austria on Friday announced separately that they had frozen the assets of more than a dozen Ukrainians connected with the political turmoil in the country.

Judicial authorities in Switzerland, a country long considered a financial haven, said they had launched criminal investigation into alleged money laundering by ousted leader Yanukovych and his son.

"A penal investigation for severe money laundering is currently being conducted in Geneva against Viktor Yanukovich and his son Oleksandr," a statement released by the prosecutor's office said.

The office added that prosecutor Yves Bertossa and police had searched the office of a company owned by Oleksandrr Yanukovich on Thursday morning and had seized some documents.

Swiss authorities also announced they were freezing the assets of 20 Ukrainian officials, including Yanukovych, his son and a number of former ministers.

Austria has also taken action with its foreign ministry announcing it has frozen assets of 18 Ukrainians suspected of violating human rights and of involvement in corruption.

"With this national decision the federal government is responding to a concrete request from the new Ukrainian government," Foreign Minister Sebastian Kurz said in a statement to the Austria Press Agency, without naming the individuals identified.

The ministry said the step was a precautionary measure until the European Union discusses potential sanctions.

The actions by Austria and Switzerland follows a decision by the European Union on February 20 to enact a travel ban and asset freeze on Ukrainians with "blood on their hands."

hc/pfd (Reuters, AP, dpa)