When President Barack Obama and other world leaders gather in Germany next week, Russia's Vladimir Putin will be left off the guest list, part of his punishment for more than a year of alleged Kremlin-supported aggression in Ukraine.

But despite vows from Obama and his European counterparts to isolate Putin as long as the crisis in Ukraine remains unresolved, the Russian leader is still a central player in major international affairs, including the U.S.-led nuclear talks with Iran.

Germany's foreign minister warned on Friday that the situation in Ukraine's separatist east was turning more "fragile" and that negotiators must focus their efforts on averting further deterioration.

Frank-Walter Steinmeier flew into Kiev just days before Ukraine's Western-backed President Petro Poroshenko's personal envoy meets pro-Russian negotiators in the Belarussian capital Minsk for talks aimed at salvaging a February ceasefire agreement.

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." 

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."

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.

Four pro-Russian rebels, a Ukrainian soldier and one civilian were killed in clashes that broke out over the past 24 hours in violation of a February truce deal, officials said Tuesday.

The civilian died during an artillery fire attack on the Avdiivka Coke and Chemical Plant, which sits on the outskirts of the rebels' de facto capital Donetsk.

Ukrainian security agents on Monday seized documents from the Kiev branches of Italy's UniCredit bank as part of an anti-terror investigation linked to individuals with accounts in the European giant.

A spokeswoman for Ukraine's State Security Service (SBU) told Interfax the snap inspections were conducted "in order to halt possible operations aimed at financing terrorist organizations" in the separatist east.

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.

One Ukrainian soldier and one pro-Russian rebel have been killed in fighting between government forces and separatists in eastern Ukraine, officials said on Monday.

Kiev military spokesman Andriy Lysenko said the serviceman was killed in shelling near the strategically important southeastern port city of Mariupol, which militants have tried to seize on repeated occasions since late August 2014.

A pro-Kiev official in eastern Ukraine said Sunday that swathes of the rebel-run region have lost access to water because fighting has damaged a water main.

The disputed Lugansk region's governor said that most villages and towns in the southwest of the industrial province were affected, but did not specify how many homes were hit.