Crisis-hit Ukraine won a vital reprieve from Russia on Monday when Moscow pushed back a possible cut in gas shipments, which would also impact parts of Europe, until next week.

Russia's surprise decision came hours before the two sides were to lock horns in Brussels over a price dispute that emerged when Moscow cancelled the discounts it awarded the pro-Kremlin regime which was ousted in February street protests.

The Organization for Security and Cooperation in Europe said on Sunday it still had no news from two teams detained in eastern Ukraine by separatist gunmen despite reports of their release.

The Vienna-based security body said both a four-member international team nabbed in the Donetsk region on Monday and another four European monitors and their local translator abducted in neighboring Lugansk on Thursday remained out of contact.

Pope Francis prayed Sunday for victims of violence in the Central African Republic, where 27 people were kidnapped this week in an attack on a church which left 17 others dead.

He also called for greater efforts to be made to secure dialogue and peace in Ukraine, where an increasingly volatile conflict has seen European monitors detained by rebels in the country's restive east.

Ukraine accused Russia of unleashing a massive campaign to persuade global powers not to recognize polls that gave the presidency to a pro-Western tycoon as both sides readied for a new round of talks on disputed gas prices.

The United States for its part acknowledged a "fundamental disagreement" with Russia and said President Barack Obama would extend his support to Petro Poroshenko when he meets the winner of the May 25 election in Warsaw on Wednesday.

Ukraine accused Russia on Saturday of unleashing a global propaganda campaign to persuade global powers not to recognize an election that gave the presidency to a pro-Western tycoon.

Washington for its part admitted to a "fundamental disagreement" with Russia and said President Barack Obama would meet Petro Poroshenko, whose May 25 presidential election win Moscow refuses to acknowledge, in Warsaw on Wednesday.

U.S. President Barack Obama will meet Ukraine's president-elect next week during a European tour aimed at shoring up regional security amid reports that a second team of European monitors went missing in the country's restive east.

The meeting in Warsaw will come less than two weeks after pro-European Petro Poroshenko, a chocolate tycoon, was elected in the shadow of a showdown between Washington and Moscow over the fate of Ukraine that has brought relations to their lowest level since the Cold War.

U.S. lawmakers urged France to break its contract to sell two warships to Russia and instead sell or lease them to NATO, which said Friday it was up to Paris to decide.

Three congressmen led by Eliot Engel, the top Democrat on the House Foreign Affairs Committee, wrote to the transatlantic alliance's Secretary General Anders Fogh Rasmussen expressing concern about the construction and sale to the Russian navy of two Mistral-class helicopter carriers.

U.S. President Barack Obama will meet Ukrainian president-elect Petro Poroshenko in Poland on Wednesday, a U.S. official said.

The meeting will take place during a trip by Obama -- also including stops in Belgium and France -- designed to underscore the U.S. security commitment to eastern Europe following Russia's annexation of Crimea, said U.S. Deputy National Security Adviser Ben Rhodes.

NATO said Friday it has seen signs Russia is pulling troops back from its Ukraine border and announced its first talks with Moscow since its disputed annexation of Crimea.

"We have seen some signs of a start of Russian withdrawal," the alliance's outgoing Secretary General Anders Fogh Rasmussen said in Vilnius.

Ukraine has paid Russia part of its gas debt, and talks will continue next week to avert a gas shutdown to the crisis-hit country, EU Energy Commissioner Guenther Oettinger said Friday.

"We don't have a final deal yet but we have made progress," he said after mediating between Russian Energy Minister Alexander Novak and his Ukrainian counterpart Yuriy Prodan, as well as both countries' energy companies, in Berlin.