NATO should beef up its maritime arm in response to the rising threat from an increasingly "bold" and "aggressive" Russia, commanders said on Wednesday.

The 28-member defense alliance's naval forces have in recent years focused on issues such as counter-terrorism and piracy, but speakers at a London conference said the group needed to prepare more to counter Russian military ambitions.

Ukrainian President Petro Poroshenko on Wednesday proposed constitutional changes designed to give broader powers to the regions, but failed to address the demands of pro-Russian fighters in the separatist east.

Poroshenko's new vision of the ex-Soviet state's basic law would trim presidential power over the provinces and extend to local towns and councils the right to oversee how their tax revenues are spent.

Ukraine announced on Tuesday the immediate suspension of all Russian gas purchases following a breakdown of EU-mediated price negotiations aimed at keeping supplies running for the next three to six months.

The state energy company Naftogaz said it would still continue transporting Russian gas supplies westward to its other European customers.

Dutch prosecutors said Tuesday they have identified many "persons of interest" in their probe into the shooting down of flight MH17 over Ukraine, but no definite suspects.

"We have a large group of people that we're at this stage branding 'persons of interest' in whom we have an above average interest," Dutch chief prosecutor Fred Westerbeke told a press briefing in Rotterdam.

A U.S. warship arrived Sunday in Georgia's Black Sea port of Batumi on a training mission, the U.S. embassy said, sending a message of support to the NATO aspirant amid the conflict in Ukraine.

The U.S. 6th Fleet's guided missile destroyer USS Laboon "will conduct routine combined training with the Georgian Coast Guard," the U.S. embassy in Tbilisi said.

A Russian court on Thursday arrested a dual Russian-Lithuanian national on treason charges, in a classified case whose details remain unknown.

"Yevgeny Mataitis has been arrested today by the Lefortovo District Court" on charges of state treason, court spokeswoman Yulia Skotnikova told AFP.

NATO head Jens Stoltenberg warned Thursday there was still a risk of heavy fighting in Ukraine and urged Russia to halt its support for pro-Moscow rebels.

In recent weeks there has been an upsurge in fighting in a conflict which has already killed some 6,500 people, despite a February ceasefire brokered in Minsk which has been regularly breached.

Japanese Prime Minister Shinzo Abe called on Russian President Vladimir Putin Wednesday to play a constructive role in finding "peaceful and diplomatic solutions" to the crisis in Ukraine, after the EU agreed to prolong sanctions against Russia.

During a 30-minute evening teleconference, Abe requested that Putin implement fully the Ukraine ceasefire deal agreed in February, Japan's foreign affairs ministry said in a statement.

The U.S. will pre-position heavy weapons, including tanks, infantry fighting vehicles and artillery in central and eastern Europe for the first time, U.S. Defense Secretary Ashton Carter said Tuesday in Estonia's capital Tallinn.

"We will temporarily stage one armored brigade combat team's vehicles and associated equipment in countries in central and eastern Europe," Carter said at a joint press conference with three Baltic defense ministers.

Ukrainian rebels Tuesday accused government forces of killing three civilians in attacks launched hours before the start of talks in Paris on ways to halt the 15-month separatist war.

Kiev's Western-backed military command reported the death of one soldier and accused the pro-Russian rebels of launching the heaviest wave of mortar and artillery fire since the start of the year in one relatively peaceful region.