Germany's foreign minister warned Friday that the situation in Ukraine's separatist east was turning more "fragile" and required all sides to focus their efforts on salvaging a three-month truce.

Frank-Walter Steinmeier flew into Kiev just days before Ukraine's Western-backed President Petro Poroshenko's peace envoy meets pro-Russian rebel negotiators in Minsk, Belarus in a desperate bid to keep all-out warfare from resuming on the European Union's eastern frontier.

In February a ceasefire agreement was reached in Minsk during marathon talks between the leaders of Germany and France, Poroshenko and Russia's Vladimir Putin. It marked the sides' second attempt at halting a 13-month war that has killed 6,300 and plunged East-West relations to a post-Cold War low.

The truce succeeded in containing the fighting but failed to put an end to daily clashes around some of the most disputed hotspots.

"The situation in east Ukraine is once again becoming fragile," Steinmeier told reporters after talks with Ukrainian Prime Minister Arseniy Yatsenyuk.

Germany's top diplomat said the sides must now consider "all options" to avoid an even further deterioration of security in a war zone that was once seen as the industrial heart of Ukraine.

A Friday phone call between Putin and the German and French heads of state appeared only to highlight the sharp differences Moscow still has with the West over what the future of the ex-Soviet nation should hold.

Russia's emphatic denial of direct involvement in the war has stymied repeated Western attempts to establish an open dialogue with Putin that could calm security fears across eastern Europe and nudge Ukraine out of political and economic crisis.

The Kremlin said Putin pointed out to German Chancellor Angela Merkel and French President Francois Hollande that "attacks by Ukrainian security forces on civilian targets that kill peaceful residents were becoming more frequent".

Hollande's office in turn said that he and Merkel stressed to Putin "the need for rapid progress in the implementation of all the measures adopted in Minsk".

- Battle for government port -

The worst fighting has most recently gripped the eastern outskirts of the strategic pro-Kiev port of Mariupol and the Donetsk International Airport -- captured by the insurgents after months of attacks at the start of the year.

An AFP team saw government forces stationed about three kilometres (two miles) from the devastated transit hub come under heavy rocket-propelled grenade attack on Thursday evening.

The Ukrainian forces responded with their own heavy weapons fire and the clashes raged through the night and into Friday morning.

Kiev on Friday reported the death of one Ukrainian soldier and a "volunteer" who supported the government troops.

"The likelihood that military activities will intensify is high," Ukrainian Defence Minister Stepan Poltorak said.

- 'They are lying' -

Yatsenyuk on Friday openly derided Russia's attempts to pin the latest violence on the pro-European team that rose to power in Kiev after the bloody ouster of Moscow-backed president Viktor Yanukovych in February 2014.

"Russia is categorically refusing to allow Ukraine to restore control over its national border," Yatsenyuk said after his talks with Steinmeier.

"Russia's (claim) that Ukraine and the West are supposedly failing to fulfil the Minsk agreements are completely groundless," said Yatsenyuk.

"They are lying -- just like they always do."

Ukraine this month captured two fighters in the east who identified themselves as active members of the Russian armed forces. Both reported performing a special reconnaissance mission ordered by their commanders.

Putin argues that Russians fighting alongside the Ukrainian militants are either "volunteers" or off-duty soldiers who personally decided to join the war.