Top diplomats from Ukraine, Russia, the United States and European Union are to meet in Geneva Thursday to try to defuse what has become one of the worst European security crisis in decades.

But as tensions soar to dangerous levels in Ukraine, threatened with a split between its Russian-speaking east and EU-leaning west, how far is Russia prepared to go to assert control over the country and what options do Western countries and Kiev have to counter this?

NATO chief Anders Fogh Rasmussen urged Russia Tuesday to de-escalate tensions over Ukraine and withdraw troops massed on the border with its Soviet-era satellite.

"I am deeply concerned by the latest developments in Ukraine," Rasmussen said, citing the seizure of government buildings in the east of the country by pro-Moscow militias.

The United Nations urged the Ukrainian authorities Tuesday to take urgent measures to reduce ethnic tensions in eastern Ukraine, but said there was no evidence of widespread attacks against ethnic Russians.

Moscow has repeatedly accused Ukraine's government of failing to protect the country's ethnic Russian minority, and used this to justify its annexation of Crimea.

Russian foreign minister Sergei Lavrov on Tuesday warned Kiev against using force to quell pro-Moscow separatists in eastern Ukraine, saying the "criminal" act would undermine talks planned in Geneva.

The four-way meeting set for Thursday involving top diplomats from Russia, Ukraine, the United States and the European Union is the latest step in a flurry of diplomacy aimed at easing the worst European security crisis in decades.

An armored column and buses filled with Ukrainian special forces stood Tuesday on a road leading to the eastern city of Slavyansk where pro-Kremlin gunmen had seized government building, as Moscow warned that Ukraine was “on the brink of civil war.”

An Agence France Presse reporter said the column of 20 tanks and armored personnel carriers was positioned in Izyum, about 40 kilometers (25 miles) north of the city and flying the Ukrainian flag.

Austrian prosecutors have launched an investigation into members of ousted Ukrainian president Viktor Yanukovych's government on charges including money-laundering, officials said Monday.

Although the names and positions of those under investigation have not been released, the investigation will look at those who were close to Yanukovych, who fled into exile in Russia after a four-month uprising in the Ukrainian capital Kiev.

EU foreign ministers agreed Monday to expand the list of those hit with sanctions for their role in the Ukraine crisis but stopped short of harsher measures ahead of a Geneva meeting of top EU, U.S., Russian and Ukrainian officials this week.

"In light of events, we decided to expand the list of those subject to asset freezes and visa bans," EU foreign affairs head Catherine Ashton said.

President Francois Hollande of France and Barack Obama of the U.S. share the same "concern" over the violence witnessed in east Ukraine over the past few days, the French presidency said in a statement Monday.

The two leaders voiced their worry in a telephone conversation in which Hollande expressed hope that a Geneva meeting on Thursday between representatives from Ukraine, Russia, the United States and Europe "will start a dialogue" to defuse the situation.

Romanian President Traian Basescu toured a U.S. warship stationed at the Black Sea port of Constanta on Monday, thanking the country's NATO allies for their "solidarity" in the face of Russia's actions in neighboring Ukraine.

Basescu visited the guided missile destroyer USS Donald Cook, stationed in Constanta along with French reconnaissance ship Dupuy de Lome after the eruption of the Ukrainian crisis.

The European Union is prepared if needed to call a Ukraine crisis summit next week to toughen sanctions against Russia, France's Foreign Minister Laurent Fabius said Monday.

Fabius said after speaking with his U.S., Russian and German counterparts that "there are actions of great organised violence" in eastern Ukraine.