NATO believes that Russia has supplied more troops and weapons to pro-Russian separatists in east Ukraine, a German newspaper reported on Sunday.

"We have noticed again support for the separatists, with weapons, troops and training. Russia is still sending troops and arms from one side of the open border with Ukraine to the other," a NATO official, who was not named, told the Frankfurter Allgemeine Zeitung.

Controversial laws designed to leave Ukraine's Soviet past behind could stoke fresh tension in the war-divided country as rebels in the east voice outrage and analysts dub the bills "crude" or even "asinine."

"In the West, friends of Ukraine will have a difficult time accepting both the wisdom and timing of such a facile and asinine decree," said historian David Marples of Canada's Alberta University.

The Nordic countries pledged Friday to step up military cooperation in a declaration which referred to Russia as the "biggest challenge to European security".

The defense ministers of Norway, Sweden, Denmark and Finland and Iceland's foreign minister said they were responding to a changed security situation caused by Russia's involvement in the Ukraine conflict and increased Russian military activity in the Baltic region.

Spain on Thursday defended EU sanctions on Russia over Ukraine, rejecting repeated criticism of the measures by Greek Prime Minister Alexis Tsipras.

During a joint news conference with Russian President Vladimir Putin in Moscow on Wednesday, Tsipras said the European Union needed "to leave behind this vicious cycle of sanctions".

Ukrainian soldiers fighting pro-Russian separatists in east Ukraine are also fighting to ensure the security of Europe, Poland's President Bronislaw Komorowski said in a speech to the Kiev parliament Thursday.

"Today when the sons of Ukraine are killed in eastern Ukraine protecting their independence, they also protect Europe. They protect it from the return of imperial thinking, from decisions that threaten the whole of Europe," he said.

Amnesty International on Thursday said it had new proof of the "summary killings" of Ukrainian soldiers captured by pro-Russian separatist fighters in eastern Ukraine, and demanded a quick impartial probe.

The rights group notably cited the case of Ukrainian soldier Igor Branovytsky, who was taken prisoner during fighting at Donetsk airport, saying he was tortured and killed while in captivity.

Ukrainian pro-Russian separatist leader Alexander Zakharchenko accused the Kiev government of "totally ignoring" the terms of a peace deal and warned in an interview with AFP that fighting might resume.

The warning came as international monitors overseeing a fragile February truce -- which is largely holding despite isolated clashes -- reported "an increase in the number of ceasefire violations in the Donetsk region" where Zakharchenko's forces are in control.

The head of the OSCE's parliamentary assembly Wednesday demanded that both Ukraine's pro-Russian separatists and its regular army stop intimidating or restricting the movements of the organization’s 400 monitors in east Ukraine.

A statement from the assembly's Finnish president Ilkka Kanerva said separatist fighters "have threatened OSCE staff on the ground" but that he particularly deplored an incident Tuesday when fighters from the self-proclaimed "Donetsk People's Republic" fired at an OSCE patrol.

Greek Prime Minister Alexis Tsipras on Wednesday called for an end to the "vicious cycle" of sanctions against Russia over Ukraine, while Russia's President Vladimir Putin denied using Athens to sow discord within the EU.

Putin and Tsipras hailed mutual ties after meeting in the Kremlin on the first of two days of the Greek premier's visit to Russia that is closely watched by the European Union amid its worst standoff with Moscow since the Cold War.

As a tenuous ceasefire brings a lull to Ukraine's yearlong conflict between pro-Russian rebels and Ukrainian troops, the lurking danger of landmines threatens lives as well as economic recovery, particularly on once rich agricultural lands.

"There are plenty of fields here where it's best not to walk, they're peppered with mines and unexploded shells," said Nikolai Utrimenko who runs a farm in Dmitrivka in separatist-controlled eastern Ukraine.