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.

Poles on Friday marked the fifth anniversary of the deadly plane crash in Russia that killed then president Lech Kaczynski and 95 officials, amid continued controversy over the causes.

With the May 10 presidential elections just around the corner and fresh leaks of cockpit transcripts, opposition politicians allege the accident was a political assassination that the government is trying to hush up.

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.

Defense Secretary Ashton Carter promised Friday that the U.S. would deploy state of the art weaponry in Asia, including the latest stealth bombers and cyber warfare units, to counter threats posed by the likes of North Korea.

"Our newest and best things are being deployed to this part of the world," Carter said in Seoul -- the second leg of a visit to the two key U.S. military allies in the region, Japan and South Korea.

Greece's avowedly atheist Prime Minister Alexis Tsipras held a tense meeting with the powerful head of Russia's Orthodox Church on Thursday as he wrapped up a trip aimed at closer relations with Moscow.

Tsipras greeted Patriarch Kirill, who enjoys a close relationship with Russian strongman President Vladimir Putin, with a strained smile at his official monastery residence following a two-day Kremlin charm offensive.

Russian President Vladimir Putin will meet Palestinian leader Mahmoud Abbas in Moscow on Monday where the issue of Middle East peace talks will be raised, Russian authorities announced Thursday.

"The two leaders will hold talks concerning key aspects of Russian-Palestinian relations and their future, with particular attention on the trade, economy and humanitarian sectors," the Kremlin said in a statement.

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.