A team of international monitors with the OSCE was detained at a checkpoint in restive eastern Ukraine on a day of deadly fighting between government forces and pro-Russian gunmen, officials said Tuesday.

The Organization for Security and Cooperation in Europe said it had lost contact with the four-member team as they were on patrol in the main rebel-held city of Donetsk on Monday.

As a short burst of shelling and machine gun fire subsides somewhere in the middle distance, Lera Meteiko crouches by the roadside with a few plastic bags of her possessions.

She had watched from her apartment building overlooking Donetsk airport as Ukrainian government helicopters on Monday battled rebel fighters with bazookas and Kalashnikovs in what was until then a peaceful suburban neighborhood.

Secretary of State John Kerry on Monday said the United States will back the new Ukraine leader and support its people, just as Petro Poroshenko was named winner of the weekend's presidential election.

Ukraine's election commission chief said that the tycoon won the election in the first round by collecting more than half the votes in a poll that Washington hopes will bring stability to the eastern European country.

Russia said Monday it was ready to talk to Ukraine's new president Petro Poroshenko, raising hopes that after months of tensions Moscow sees in the billionaire a leader it can work with.

"We are ready for dialogue with Kiev's representatives, with Petro Poroshenko," Foreign Minister Sergei Lavrov said in Russia's first reaction to his victory in Sunday's vote.

Billionaire oligarch Petro Poroshenko said Monday he would not let rebel-held east Ukraine become another Somalia as he prepared to take over the presidency with a clear signal from Moscow that it is ready to work with his government.

Poroshenko has moved swiftly to stamp his authority as Ukraine's new leader after claiming victory in Sunday's presidential election, with partial results showing he trounced his rivals in the first round.

The main airport in Ukraine's eastern city of Donetsk was rocked by an explosion and heavy shooting on Monday after armed rebels seized the facility, an AFP correspondent at the scene said.

Thick black smoke was seen rising from the airport complex as the sound of heavy machinegun fire rang out and fighter jets could be heard overhead.

U.S. President Barack Obama hailed "courageous" Ukrainians who went to the polls Sunday in a presidential vote that Washington and the West hope will bring stability back to the country after months of turmoil.

"Despite provocations and violence, millions of Ukrainians went to the polls throughout the country," the White House quoted Obama as saying.

Chocolate magnate Petro Poroshenko appeared poised to lead Ukraine as it grapples with its worst crisis since independence after exit polls showed him winning Sunday's presidential election outright.

He won almost 56 percent of the vote against almost 13 percent for former prime minister Yulia Tymoshenko, according to a joint survey conducting by Ukraine's three leading polling agencies.

Several hundred Ukrainians voted in the country's presidential election in Moscow on Sunday but the vast majority of their countrymen in the Russian capital stayed away.

Viktor Girzhov, the head of the electoral commission at the Ukrainian embassy, said that 667 people had voted as of 4:00 pm Moscow time (1200 GMT).

Ukraine on Sunday called Russian Prime Minister Dmitry Medvedev's surprise visit to annexed Crimea a "deliberate provocation" aimed at destabilizing the country on election day.

"A visit by the Russian prime minister to occupied Ukraine on the day of the presidential election in Ukraine is particular impudence and a deliberate provocation aimed at destabilizing the situation in Ukraine," the foreign ministry said in a statement issued moments after Medvedev's arrival in Ukraine's annexed Black Sea peninsula.