Russian President Vladimir Putin told U.N. Secretary General Ban Ki-moon on Tuesday he wanted clear United Nations condemnation of the "anti-constitutional" military operation by Ukraine against separatists in the east of the country.

Putin told Ban in telephone talks that Russia "expects clear condemnation from the United Nations and the international community of the anti-constitutional actions" by Ukraine, which according to Putin had sharply worsened the crisis in the country, the Kremlin said in a statement.

Russia carried out a new test-launch of an intercontinental ballistic missile (ICBM) as tensions over Ukraine escalated, the Pentagon confirmed on Tuesday.

Russia informed U.S. authorities of the test prior to the launch on Monday, in keeping with an existing weapons treaty, Pentagon spokesman colonel Steven Warren said.

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.

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.

A Russian fighter jet made several passes at low altitude near a U.S. destroyer cruising in international waters in the Black Sea at the weekend, the Pentagon said Monday, branding it "provocative and unprofessional."

The incident close to the Romanian coast further heightens tensions already inflamed by Russia's actions in Ukraine, where Moscow annexed the Crimean peninsula and stands accused of stoking unrest.

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.