Ukrainian President Petro Poroshenko warned Thursday of the "colossal threat" of major clashes in eastern Ukraine where at least 26 people have died in renewed fighting between government forces and pro-Russian rebels.

Fresh clashes erupted near the rebel stronghold of Donetsk on Wednesday, in one of the worst outbreaks of violence since a European-brokered truce was agreed in February.

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."

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.

Amnesty International on Friday accused Ukrainian forces of torture and pro-Russian rebels of even more serious war crimes such as summary executions committed both before and after a February truce deal.

The damning report adds to a growing string of independent findings suggesting that Europe is witnessing the most brutal conflict on its outer border since the 1990s Balkans crises.

Kiev on Tuesday showed off two purported Russian soldiers it captured during a gun battle in the separatist east that Ukraine's pro-Western leadership says proves the Kremlin's direct involvement in the war.

The two wounded men -- recovering in a Kiev military hospital under the guard of masked state security men -- have turned into pawns in a bitter public relations battle being waged by Moscow and Kiev since Saturday's firefight.

Pro-Russian separatists appear to be making preparations for a fresh offensive in eastern Ukraine, U.S. Defense Secretary Ashton Carter told lawmakers on Wednesday.

The Pentagon chief also said that sanctions imposed by European countries and a decline in oil prices were the most effective way to exert pressure on Russia over its role in the conflict.

The Ukrainian presidency on Thursday said Russian President Vladimir Putin was open to the deployment of a peacekeeping mission in war-torn eastern Ukraine, after the leaders of both countries spoke by phone with their French and German counterparts.

"In the context of the discussion of ways towards a peaceful resolution (of the conflict), President Putin has accepted the possibility of deploying a peacekeeping contingent to the Donbass region," the presidency said in a statement.

Russia on Friday charged Ukrainian airforce officer Nadia Savchenko over the deaths of two Russian journalists in a politically charged case that has become emblematic of tensions between Kiev and Moscow.

The 33-year-old helicopter navigator is charged with acting as an accessory in the murder of two Russian journalists "with motives of political hatred."

British Prime Minister David Cameron is establishing a fund to help Eastern European states resist pressure from Russia, several media outlets reported on Friday.

Sky News said the "Good Governance Fund" would amount to £20 million ($30 million, 28 million euros) in 2015 and 2016.

Russia said Thursday it had doubled the number of troops taking part in mass drills ordered by President Vladimir Putin this week to 80,000 in a major show of strength amid tensions with the West over Ukraine.

Putin on Monday ordered drills for more than 40,000 troops in regions spanning the country, from the Arctic to the far east to the volatile southern Caucasus, and ordered nuclear bomber jets to be deployed in Crimea a year after its annexation by Moscow.