Ukraine will demand that Russia cease its support for "terrorist activities" on its territory, Kiev's foreign minister said Wednesday on the eve of crunch talks with Moscow, the U.S. and EU.

"Our main demand is to de-escalate the situation in eastern Ukraine. We want Russia to withdraw the troops from the eastern borders of Ukraine. We want Russia not to support terrorist activities in eastern Ukraine," Foreign Minister Andriy Deshchytsya said after he landed in Geneva for Thursday's meeting.

The United States warned Russia Wednesday to stop its "provocation" in eastern Ukraine, reiterating that new sanctions could be slapped on the Kremlin.

With U.S. Secretary of State John Kerry en route to Europe for international talks on the most serious East-West crisis in years, and a Ukrainian effort to reassert control over its eastern regions floundering, the State Department demanded de-escalation and demobilization on the part of Russian forces.

It's a welcome more suited for rock stars and matinee idols, not necessarily black-masked soldiers in camouflage here to protect a rebellion.

"You're our heroes! We love you!" the crowd clamors, smartphones in hand to capture history in this eastern Ukrainian town of Slavyansk with a population of 140,000, pro-Moscow Russian speakers in their vast majority.

U.S. senator John McCain on Wednesday described the West's response to Russian actions in Ukraine as "almost laughable", saying a stronger stance is needed to keep the Kremlin in check.

The recent U.S. and EU sanctions against Russia were "the weakest response imaginable", the Republican senator told reporters in the Lithuanian capital Vilnius while on a tour of the Baltic states.

U.S. Secretary of State John Kerry left Wednesday for Geneva for high-stakes talks on Ukraine, armed with the threat of more sanctions against Moscow if diplomacy fails, as tensions on the ground escalated sharply.

The U.S. and the European Union will hold their first four-way talks with Ukraine and Russia on Thursday to address a worsening crisis, although U.S. officials have set low expectations for the meeting, still smarting from a slew of failures in past weeks.

NATO said Wednesday it will deploy additional air, sea and land forces in eastern Europe in response to the worsening crisis in Ukraine and take further action if needed.

"Today we have agreed a package of military measures," NATO head Anders Fogh Rasmussen said after a meeting of ambassadors of all 28 members of the transatlantic alliance.

Ukrainian and pro-Russian forces flexed their military muscles in the restive east of the country on Wednesday, a day ahead of high-level diplomatic talks on the escalating crisis.

Ukraine's defense ministry said that pro-Russian militants had seized six armored vehicles dispatched by Kiev to the eastern town of Kramatorsk to quell a separatist insurgency.

Economic sanctions by Europe against Russia "would destroy Cyprus's economy," Nicosia's foreign minister Ioannis Kasoulides said in a German newspaper interview Wednesday.

"There are very strong economic ties between Cyprus and Russia. If sanctions are really necessary, then every member state should decide for itself whether to take part," Kasoulides said.

Russian leader Vladimir Putin warned that Ukraine is on the verge of civil war, the Kremlin said Wednesday, after the Kiev government sent in troops against pro-Moscow separatists in the east of the country.

"The Russian president remarked that the sharp escalation of the conflict has placed the country, in effect, on the verge of civil war," the Kremlin said in a statement on telephone talks between Putin and German Chancellor Angela Merkel.

The United States is coordinating with its European allies on "additional steps" to slap more sanctions on Russia over the crisis in Ukraine, a U.S. official said Tuesday.

Secretary of State John Kerry spoke by phone with his French, German and British counterparts Tuesday, but new sanctions are unlikely to be unveiled before crunch talks slated for Thursday in Geneva, State Department spokeswoman Jen Psaki told reporters.