NATO head Jens Stoltenberg said Wednesday he was "disappointed" by Russia's decision to quit the landmark Treaty on Conventional Forces in Europe as arms control efforts remained important to all sides.

"We are disappointed by the Russian decision to suspend participation in the joint consultative group of the CFE," Stoltenberg told a briefing at NATO's European military HQ in the Belgian city of Mons.

A former Chechen police officer who admitted taking part in the murder of Russian opposition activist Boris Nemtsov "likely confessed under torture", a member of the Kremlin's human rights council told AFP on Wednesday.

Zaur Dadayev allegedly confessed and was charged but has now insisted to the council's Andrei Babushkin that he is "innocent" and only made the admission under duress.

Polish and Latvian officials on Tuesday voiced skepticism over calls for a European Union army to counter a militarily resurgent Russia.

"It's a very risky idea," Polish Foreign Minister Grzegorz Schetyna told Poland's private Radio Zet in reaction to Sunday's proposal by European Commission head Jean-Claude Juncker.

The top U.S. diplomat for Europe declared Tuesday that Crimea and parts of eastern Ukraine were subject to a "reign of terror", blaming Russia for a conflict that has claimed thousands of lives.

"Even as Ukraine is building a peaceful, democratic, independent nation across 93 percent of its territory, Crimea and parts of eastern Ukraine are suffering a reign of terror," Assistant Secretary of State Victoria Nuland told the Senate Foreign Relations Committee.

Both sides in the conflict in eastern Ukraine have withdrawn most of their heavy weapons from the frontline, bolstering a month-old truce, Ukrainian President Petro Poroshenko has said.

In an interview with state television Monday evening Poroshenko said government forces had "withdrawn the lion's share of multiple launch rocket systems and heavy artillery."

Poland on Tuesday criticized a Swiss company's sale of high-tech camouflage netting to Russia despite Western sanctions over its role in the Ukraine conflict.

"It must be condemned... supplying this kind of equipment facilitates the aggression of the Russian military," Foreign Minister Grzegorz Schetyna told PAP news agency of the 85 million euros ($91 million sale).

A total of 64 soldiers have been killed in eastern Ukraine since a ceasefire deal between Kiev and pro-Russian rebels went into effect on February 15, President Petro Poroshenko said Monday.

While the shaky truce has been largely holding, sporadic clashes have continued with both sides accusing the other of violating the European-brokered, Russian-backed peace deal signed in the Belarussian capital last month.

Ukraine accused pro-Russian separatists Monday of using mortars and a tank to fire on government positions near the eastern port of Mariupol in clashes that lasted several hours, violating a nearly month old ceasefire.

The militants had fired on Ukraine's positions and were attempting to "force our contingents from Shyrokyne," a village about 10 kilometers east of Mariupol on the Azov Sea coast, the headquarters of the army's operations in the east said in a Facebook post.

Russia has vastly enhanced its military capabilities in Crimea since annexing the peninsula, turning it into a "power projection platform," a senior NATO general was reported Monday as saying.

"What we've seen is easy to describe as the militarization of Crimea," the alliance's top commander for Europe, General Philip Breedlove, said in an interview with Ukrainian channel 1+1.

A friend of slain Russian opposition activist Boris Nemtsov on Monday dismissed theories that radical Islamists were responsible for gunning him down as "absurd" and politically motivated.

Investigators have said they were looking into the possibility that the former deputy prime minister was killed over his support for French satirical magazine Charlie Hebdo which published images of the Prophet Mohammed.