The United States, the European Union, Russia and Ukraine will meet at ministerial level next week to discuss the Ukraine crisis, the EU said on Tuesday.
The talks are aimed at breaking the impasse since Russia annexed the southern Ukrainian peninsula of Crimea in February. Russian troops are now massed along the borders of the two countries.
The meeting will involve U.S. Secretary of State John Kerry, EU foreign policy chief Catherine Ashton, Russian Foreign Minister Sergei Lavrov and Ukraine\’s Foreign Minister Andriy Deshchytsia, the EU said.
Further details of the meeting, which will be held at an unspecified location in Europe, are still being worked on, an EU source said.
Ukraine\’s interior minister has warned pro-Russian activists who have taken over state buildings in eastern cities to enter talks to find a political solution or face "force".
Arsen Avakov said the situation would "be resolved in 48 hours" either way.
"I want to repeat that there are two options: political settlement through negotiations and the use of force," Avakov told reporters. "We are ready for both options."
Avakov was speaking as anti-government protesters in Luhansk erected high barricades along a thoroughfare running in front of the security service premises.
Earlier, some of those who had been inside security service offices in Luhansk since Sunday left the building.
Tensions have been high since government buildings in the eastern cities of Luhansk, Donetsk and Kharkiv were taken over by pro-Russian activists on Sunday.
Meanwhile, Russia has threatened that its gas company Gazprom could ask Ukraine to pay for gas in advance, according to Russian state news agencies.
In a meeting on Wednesday with government ministers, Putin said that asking for advance payments "corresponded with the contract" between Ukraine and Moscow. But he asked that Gazprom refrain from such drastic measures until "additional consultations" between both sides.
A U.S. State Department spokeswoman says the "evidence is overwhelming" that recent pro-Russia uprisings in eastern Ukrainian cities were a "very carefully orchestrated, well-planned, well-targeted" effort to take over buildings in several different cities in the same 24-hour period.
Victoria Nuland, the top U.S. diplomat for Europe, made the remarks when asked by a U.S. lawmaker if the U.S. has any proof that Russia was involved in the unrest.
Nuland testified Wednesday that events in Ukraine are "a wake-up call" and said "everything we have stood for over 40 years as a community of free nations" is at risk if "aggressive actions" are allowed to go unchecked and unpunished.
She said the United States does not have high expectations for upcoming four-way talks on Ukraine, but it is important to keep channels of communication open.
U.S. Secretary of State John Kerry says Russian agents have been sent to eastern Ukraine to create "chaos." Kerry also said that Russia\’s actions in eastern Ukraine could be a contrived pretext for military intervention just like in Crimea.
Kerry on told US Congress on Tuesday that the Kremlin was seeking to "create chaos" in Donetsk, Kharkiv and Luhansk as a pretext for more military intervention.
"Everything that we\’ve seen in the last 48 hours, from Russian provocateurs and agents operating in eastern Ukraine, tells us that they\’ve been sent there determined to create chaos," Kerry said, adding that he would meet next week with Russian Foreign Minister Sergei Lavrov to discuss the crisis.
Rebels occupying Donetsk\’s regional government building on Monday declared a "people\’s republic" and called for a referendum on secession from Ukraine to be held by 11 May.
There have been talks overnight in Donetsk between the authorities and pro-Russian activists who had occupied the regional administration building.
On Tuesday, Russia warned Ukraine to stop any military preparations, with the Russian foreign ministry saying in a statement that such preparations risked causing a civil war.
Russian Foreign Minister Sergei Lavrov says the southeastern parts of Ukraine should be included in talks about the country\’s future.
He told reporters Tuesday in Moscow that Russia wants to see those regions, which are largely Russian-speaking, represented in multilateral talks.
Russia recently annexed the peninsula of Crimea following a referendum.
In Paris, NATO chief Anders Fogh Rasmussen warned that further Russian intervention in Ukraine would be a "historic mistake" that would further isolate Russia from the world.
Also Tuesday, Russia\’s foreign ministry called on Ukraine to halt what it called military preparations in the southeast that could lead to civil war.
Pro-Russian demonstrators took over buildings Sunday in the Ukrainian cities of Kharkiv, Donetsk, and Luhansk. They are demanding referendums on splitting with Ukraine — the same kind of vote that took place last month in Crimea.
The White House says it has evidence that some of the protesters are not local Russian-speakers but were brought in from elsewhere and paid to start trouble.
The US and the EU have already imposed targeted sanctions on Russian and Ukrainian individuals over the annexation of Crimea.
On Monday, pro-Russian protesters occupying a regional administration building in Donetsk declared the creation of a "people\’s republic" separate from Ukraine.
"In the event of aggressive action from the illegitimate Kiev authorities, we will appeal to the Russian Federation to bring in a peacekeeping contingent," said the proclamation, voiced by an unidentified protester.
The activists later read the text to a cheering crowd of about 1,000 people outside the building.
Police in Luhansk also said protesters occupying the state security building there had seized weapons.
Russia said the Ukrainian government should stop blaming Russia for its problems.
Russia is refusing to recognise the new authorities in Kiev who took power after pro-Moscow President Viktor Yanukovych was ousted in February.
Eastern Ukraine was the political heartland of Yanukovych and has a large Russian-speaking population.
Yanukovych fled Kiev for Russia after months of street protests triggered by his refusal to sign an association agreement with the EU in favour of closer ties with Russia. More than 100 people died in the ensuing unrest.