WARSAW--The chances of a peaceful solution to the separatist conflict in eastern Ukraine are waning and poses the biggest security threat to Europe since the end of the Cold War, Poland's defense minister said Thursday.

There is "no reason to believe in Russia's good intentions" as it tries to block Ukraine's attempts to integrate with Europe, Tomasz Siemoniak said, adding the North Atlantic Treaty Organization is well prepared to tackle a potential spillover of the conflict to the Baltic states.

The alliance has contingency plans for potential events in the ex-Soviet republics that are now part of NATO, he said.

The conflict in eastern Ukraine intensified recently with the pro-Russian separatist forces on the offensive, which Kiev and western officials describe as a fresh infusion of Russian military personnel and materiel. Moscow denies any military involvement in the conflict.

"It is clear there are less and less chances for a peaceful solution to the Ukrainian crisis," Mr. Siemoniak said. "I believe some very tough months and years are ahead because Russia's clear intention is to block Ukraine's way towards Europe."

The cease-fire conditions negotiated last year in Minsk aren't being enforced and there is no new framework for the sides of the conflict to sit down to negotiate, creating a bleak outlook, the Polish defense minister said.

"When you talk, you don't fight. When you don't talk, all scenarios are possible," Mr. Siemoniak said.

Poland considers cooperation with Ukraine as its top priority and is willing to sell military equipment to the Ukrainian government under "favorable conditions" and will also accelerate the creation of a joint Polish-Lithuanian-Ukrainian brigade.

The Polish government has been a staunch supporter of Kiev in its struggle against pro-Russian rebels, although Warsaw has become less vocal since Prime Minister Ewa Kopacz took over from Donald Tusk last year. The new premier stressed that Poland's own security was the new administration's top concern.

The intensifying conflict prompted U.S. and European leaders to threaten new sanctions in the wake of the rebel rocket attack that killed dozens of Ukrainian civilians last week.

Although Mr. Siemoniak didn't offer outright support for new sanctions, he said "the climate has changed" from a few weeks ago, when some EU member states even considered easing sanctions on Russia. Poland's Foreign Minister Grzegorz Schetyna said on Thursday that sanctions should be stepped up.

Western officials had hoped Moscow would be forced to back down through a combination of sanctions and financial pain amid the plunging price of crude oil, Russia's main export.

But Mr. Putin has been unwilling to order militants to return control of the Ukraine-Russia border to Kiev, a crucial point of the September deal. Russia's Foreign Minister Sergei Lavrov said in an opinion piece published in Serbian magazine Horizons this week that the sanctions won't prompt Russia to change its course, which it sees as a "right and just" defense of its interests.

"We have repeatedly, and in various formats, warned that attempts to make Kiev choose one vector of its foreign policy--either west or east--bode most serious adverse consequences for Ukraine's still fragile statehood. We were not heard," Mr. Lavrov said in an article republished on his ministry's website.

Write to Patryk Wasilewski at patryk.wasilewski@wsj.com

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