Ukraine's new Western-backed president announced plans Wednesday to order a unilateral ceasefire in the separatist east that could help end a bloody pro-Russian insurgency and avert his ex-Soviet country's breakup.

Petro Poroshenko took a further step towards relieving tensions with Russia by deciding to replace acting Foreign Minister Andriy Deshchytsya -- a hate figure in Moscow -- with his current envoy to OSCE-mediated negotiations with the Kremlin.

But he also appealed for US and EU help to secure his country's porous border with Russia and stem the influx of arms and militants into the conflict zone.

The frontier has witnessed ferocious clashes in recent weeks in which militants armed with Russian-made rocket launchers have attacked Ukrainian border guard camps and ambushed army patrols.

A Ukrainian defence spokesman said the rebels had recently "stepped up their activities" and killed three soldiers.

But the respected Dzerkalo Tyzhnia news site cited defence sources as saying the fighting had killed 15 soldiers and left 13 others missing.

Poroshenko unveiled his peace initiative after late-night talks with Russian President Vladimir Putin in which the Kremlin chief also raised concern about the death in Lugansk fighting on Tuesday of two members of a Russian state TV crew.

This picture taken and released by Ukraine's Presidential press service on June 18, 2014 shows President Petro Poroshenko (R) speaking during celebrations marking the release of students at the National University of Defense of Ukraine in Kiev Mykola Lazarnko, Presidential Press Service/AFP

- Ceasefire order 'within days' -

"The peace plan begins with my order for a unilateral ceasefire," Poroshenko told reporters in Kiev.

"Immediately after that, we must receive support for the presidential peace plan from all sides involved (in the conflict)."

Acting Defence Minister Mykhailo Koval said the order would be issued "literally within days".

Poroshenko's plan also calls for Putin to formally recognise the new leadership in Ukraine that emerged after months of deadly pro-EU protests ousted the Russian-backed president in February.

"On the one hand, they are talking about a ceasefire. And on the other, they are continuing their aggression," Russian Foreign Minister Sergei Lavrov said.

Washington commended Ukraine for its "good-faith efforts" but State Department spokeswoman Jen Psaki added: "Naturally, they need a partner in this effort."

Psaki said the United States had seen no indication of reciprocal good-faith efforts by Moscow.

Miners march during a rally in support of the self-proclaimed "Donetsk People's Republic" in Donetsk, eastern Ukraine, on June 18, 2014 Daniel Mihailescu, AFP

"That's one of the reasons why a partner is needed, and Russia of course must support the peace plan... instead of continuing to support the separatists on the ground," Psaki said.

Poroshenko, a 48-year-old confectionery tycoon, won Ukraine's May 25 presidential election on a promise to quickly resolve the worst crisis since its independence from the Soviet Union in 1991.

Yet Poroshenko's problems have since been compounded by a cut in Russian gas supplies that threatens to plunge the country into an even deeper recession.

The head of Ukraine's state-owned gas firm Naftogaz said Kiev is seeking a common strategy with the EU to resolve the gas issue.

"It is high time to unite the efforts of different countries to resolve the issue of gas pricing in a transparent manner," Andriy Kobolev told AFP.

Poroshenko said the ceasefire was meant to be a temporary measure to give militants a chance to disarm.

People hold a Russian flag as they rally for independence for the self-proclaimed "Donetsk People's Republic" in Donetsk, eastern Ukraine, on June 18, 2014 Alexander Khudoteply, AFP

The rebels have previously rejected similar calls and vowed to continue a campaign to join Russia that a UN report said has killed at least 356 civilians and fighters on both sides.

The UN rights office said in a report that the rebels had imposed a "reign of fear, if not terror" in areas under their control.

A top commander in the self-proclaimed Donetsk People's Republic dismissed Poroshenko's initiative as "meaningless".

"We are only interested in seeing the occupying forces leave our land," Denis Pushilin told Moscow's Dozhd television.

- Major irritant -

Poroshenko's decision to tap 46-year-old Pavlo Klimkin as foreign minister will help address one of the biggest irritants in relations with Moscow.

Klimkin is a veteran diplomat who recently served as Ukraine's ambassador to Germany and is now Poroshenko's personal representative at talks with Moscow that were launched on June 8 by the Vienna-based Organisation for Security and Cooperation in Europe (OSCE).

Deshchytsya became embroiled in controversy at the weekend when he called Putin "a prick" while trying to restrain protesters who attacked Moscow's embassy compound in Kiev.

Poroshenko will personally present the candidacy of Klimkin and that of Ukraine's new prosecutor general for parliamentary approval on Thursday.

The new leader's second talks with Putin in four days and nomination as foreign minister of a man who has already won a degree of Moscow's trust come in sharp contrast to the freeze in ties that followed Russia's annexation of Crimea in March.

But Poroshenko has also stressed that he will be unable to put an end to the fighting until Ukraine regains complete control of its 2,000-kilometre (1,230-mile) land border with Russia.

Poroshenko's office said he told German Chancellor Angela Merkel on Tuesday that Western help sealing the frontier was "essential".

The United States has pledged "non-lethal military aid" such as helmets and medical supplies for Ukraine's underfunded armed forces.

But Washington has refused to provide any combat equipment and rejects the idea of deploying ground forces in Ukraine.