An unidentified assailant shot dead a pro-Russian journalist in Ukraine Thursday leading Kiev to brand the latest murders of pro-Moscow figures an enemy "provocation".

Reporter Oles Buzyna, 45, a supporter of Ukraine's ousted Kremlin-backed president Viktor Yanukovych, was gunned down in central Kiev just hours after the slaying of former ruling party lawmaker Oleg Kalashnikov in the city.

Russia can expect international sanctions against it to be lifted only when it complies with the terms of a ceasefire with Ukraine, foreign ministers from the Group of Seven said on Wednesday.

The top diplomats, gathering in Germany for their second meeting since the expulsion of Russia from their ranks last year over its annexation of Crimea, said the truce agreed in February in the Belarussian capital of Minsk must be respected.

Dutch and international investigators will renew their search for body parts and debris at the MH17 plane crash site in restive eastern Ukraine, including at a location previously considered unsafe, the team's chief announced Wednesday.

"The team is in Donetsk and ready to start tomorrow (Thursday)," Pieter Jaap Aalbersberg said at a press conference in The Hague.

Estonian Prime Minister Taavi Roivas on Tuesday urged allies to send over more troops to the region, calling on Germany to take the lead in the effort amid security fears over Russia.

"Estonia sees that a more long-term stationing of European allies in Estonia and the Baltic region could be led by Germany," Roivas said after meeting with German Defense Minister Ursula von der Leyen in Tallinn.

Three Russian ships including a destroyer have entered the English Channel and are being monitored by Britain's Royal Navy as they pass through, the Ministry of Defense said Tuesday.

It is the latest in a string of similar incidents and comes at a time of tense relations between London and Moscow over the crisis in Ukraine and the inquiry into the death of Russian ex-spy Alexander Litvinenko.

Six soldiers were killed and 12 injured in separatist-held east Ukraine in the last 24 hours as a fresh flare-up of fighting rattles a shaky ceasefire, Kiev's military said Tuesday.

The situation in the conflict zone "remains unstable", with "armed provocations by the enemy continuing on almost all sides," military spokesman Andriy Lysenko said.

Surging fighting around hotspots in separatist-held east Ukraine put fresh pressure on a shaky ceasefire under review at talks on Monday between the French, German, Russian and Ukrainian foreign ministers.

The four meet in Berlin from 1745 GMT to assess progress in the truce deal agreed in February between the Ukraine government and pro-Russian rebels to end a conflict that has killed more than 6,000 people over the past year.

German Foreign Minister Frank-Walter Steinmeier has called on Russia and Ukraine to set in motion the next phase of the shaky Minsk peace accords aimed at stopping the fighting in east Ukraine, ahead of talks about their implementation in Berlin on Monday.

"We expect both Moscow and Kiev to seize the central issue of the implementation of the next phase of Minsk," Steinmeier said in an interview in Monday's edition of Die Welt newspaper.

Estonia's President Toomas Hendrik Ilves has called for NATO to send more troops to his country to counter the threat posed by increasingly assertive neighbor Russia, a report said on Sunday.

Ilves told Britain's Daily Telegraph newspaper that Estonia was "part of a group of countries who are mentioned in a threatening way" by Russia and that the military alliance, which Estonia joined 11 years ago, should set up permanent combat units in his country.

A hospital in the rebel-controlled city of Donetsk was hit by artillery fire on Sunday, Orthodox Easter, despite a ceasefire, but caused no casualties, local authorities said.

"Four mortar shells hit hospital No 21 on Sunday morning. Three exploded in the hospital grounds and one fell on the roof but did not explode," said Ivan Prikhodko, administrative head of the Kuybishevski district in Donetsk, quoted on the official separatist website.