Russian President Vladimir Putin on Thursday banned revealing information about troop deaths in "special operations" during peacetime, as the Kremlin continues to rebuff accusations its soldiers are fighting in Ukraine. 

The strongman signed an amendment adding to what Russia classifies as "state secrets" any "information revealing losses of personnel... in peacetime during special operations." 

Russia's provocative rhetoric and its dramatic expansion of flights by nuclear bombers are deeply troubling and dangerous, NATO Secretary General Jens Stoltenberg said Wednesday.

Russia's plans to deploy nuclear-capable missiles in Kaliningrad -- near Poland's border -- and its threat to move nuclear forces in Crimea would "fundamentally change the balance of security in Europe," Stoltenberg warned, in speech during a visit to Washington.

U.S. Vice President Joe Biden on Wednesday denounced Vladimir Putin's "pure aggression" in Ukraine, warning that Western nations must be prepared to respond with further sanctions against Russia if necessary.

Condemning the Russian president's actions abroad and repression at home, Biden said facing down Putin was "critical to checking further aggression down the road."

Syrian Foreign Minister Walid Muallem said Wednesday that Iran and Russia would never give up their support for his regime in the face of a four-year-old armed revolt.

"I assure you that the relationship between Syria, the Russian Federation, and the Islamic Republic of Iran is much deeper than some people think," Muallem said at a press conference with his visiting Armenian counterpart Edward Nalbandian.

Nuclear envoys from South Korea, Japan and the United States agreed Wednesday to step up pressure -- including possible sanctions -- on North Korea to abandon nuclear weapons, while keeping diplomatic options "open."

Speaking to reporters after trilateral talks in Seoul, the three officials also highlighted the "seriousness" of the progress the North has been making with its expanding nuclear and ballistic missile programs.

U.S. President Barack Obama on Tuesday criticized Russia's "increasingly aggressive" stance on Ukraine after discussing the situation there with NATO's secretary general.

"We had a chance to discuss the situation in Ukraine and the increasingly aggressive posture that Russia has taken," Obama said after an Oval Office meeting with Jens Stoltenberg.

The leader of France's far-right National Front, Marine Le Pen, visited Moscow Tuesday for talks with the speaker of the lower house that were closed to press.

Le Pen previously met the speaker Sergei Naryshkin, Russia's fourth most important official and a member of the ruling United Russia party, in April last year.

Russia was conducting huge and unexpected war games on Tuesday, coinciding with a Nato drill in the Arctic at a time of heightened tensions in the region.

The snap check launched on the orders of President Vladimir Putin involved up to 700 units of military hardware, 12,000 troops and up to 250 aircraft, the defense ministry said.

A German deputy with strong links to Ukraine was denied access to Russian territory over the weekend, a decision slammed as "unacceptable" by Berlin on Monday.

The German foreign ministry said the government had made an official protest through the Russian embassy in Berlin.

The United States led criticism of Russia's new law targeting "undesirable" NGOs, which activists fear will stifle critical voices at international rights groups working in the country.

Under the law signed by President Vladimir Putin on Saturday evening, Russian prosecutors will be able to target foreign groups whose "undesirable activities" are deemed to threaten "state security" or the "basic values of the Russian state."