Ukraine's army said on Wednesday that a new nine-day truce with pro-Russian insurgents was holdings despite small skirmishes that claimed the lives of two government soldiers this week.

"A relative calm on the front is holding," Ukrainian military spokesman Andriy Lysenko told reporters.

Four civilians were killed in intense shelling attacks between the Ukrainian army and pro-Russian rebels in the separatist east of the war-torn state, officials from both sides said Thursday.

"Two civilians were killed as a result of shelling in Dzerzhynsk," about 50 kilometers (30 miles) north of the rebels' de facto capital Donetsk, representative of the region's Kiev-appointed administration's health department told Agence France Presse.

U.S. Secretary of State John Kerry on Monday pressed his Russian counterpart Sergei Lavrov to ensure that Moscow respects the ceasefire in eastern Ukraine, the State Department said.

In a telephone call to Lavrov, Kerry "urged Russia to seize the opportunity of upcoming meetings of the Trilateral Contact Group and its Working Groups to accelerate progress on implementing the Minsk agreements," signed in February, said John Kirby, a State Department spokesman.

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.

A top pro-Russian rebel said on Friday that a new round of Ukraine crisis talks had resulted in no breakthrough but would continue in the coming weeks.

The chief negotiator of the self-declared Lugansk People's Republic said the European-mediated talks got stuck on the crucial issue of when and under what terms Ukraine's two renegade eastern provinces could conduct their own elections.

U.S. Secretary of State John Kerry on Wednesday warned Russia that now was a critical time to implement a fragile peace deal to end the fighting in Ukraine, after his talks with President Vladimir Putin raised hopes of a slackening in tensions.

Kerry said there was an "enormous moment of opportunity" to bring to an end over a year of fighting in eastern Ukraine between government forces and pro-Russia separatists which has dragged relations between Moscow and the West to a new post-Cold War low.

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.

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.

Western leaders on Tuesday called for a "strong reaction" from the international community if a major violation of a truce in Ukraine occurs, the French presidency said after video conference talks.

The leaders of the United States, France, Germany, Britain and Italy as well as EU head Donald Tusk held video talks on the Ukraine conflict on Tuesday.

A plea from Ukraine for international peacekeepers to enforce a shattered ceasefire in the east ran into strident opposition Thursday from pro-Russian rebels and Moscow.

They argued the appeal, made late Wednesday by Ukrainian President Petro Poroshenko, sought to "destroy" the U.N.-backed truce agreed under European mediation last week in the Belarus capital Minsk.