Russian President Vladimir Putin has denied Russia is involved in the unrest in eastern Ukraine.
The Kremlin says Putin issued the denial to U.S. President Barack Obama during a telephone call on April 14.
According to the Kremlin, Putin rejected claims of Russian agents' involvement in protests as "speculations based on unreliable information."
According to the White House, Obama told Putin that Moscow would face further costs if its actions in Ukraine persisted.
Obama urged Putin to use his influence with armed, pro-Russian groups to convince them to leave buildings they have seized in eastern Ukraine.
Obama told Putin that Russia must withdraw its troops from Ukraine's border to reduce tensions, noting Ukraine's government has acted with "remarkable restraint."
The two leaders spoke as pro-Russian groups continue to occupy government buildings and security headquarters in some 10 cities and towns in eastern Ukraine.
Moscow has said it is worried Russian-speakers in Ukraine could face threats from right-wing Ukrainian nationalists. This is rejected by supporters of Ukraine's pro-Western authorities who took power after President Viktor Yanukovych fled Ukraine in late February.
The Ukrainian government has announced plans for what it calls an "antiterrorist" operation in the east to roll back the seizures of government facilities.
Western states have accused Moscow of orchestrating the unrest. They have warned of imposing new sanctions targeting sectors of the Russian economy in response to what they describe as Russia's further intervention in Ukraine.
In the eastern Ukrainian city of Slovyansk, pro-Russian protesters have declared the formation of a new "Donetsk People's Republic" and appealed April 14 for Putin to help them against Ukrainian government forces.
Ukrainian and Western officials have accused the pro-Russia movement of being led by, or even being part of, the Russian military. Western officials say the weapons, uniforms, and level of training of the pro-Russia groups indicate they are not hastily formed civil-defense forces, as claimed by supporters of the groups.
WATCH: Ukraine's foreign minister says evidence shows Russia's role.
The pro-Russia forces have so far defied the Ukrainian government's order to evacuate buildings and disperse. They have shown little concern over the Ukrainian government's vow to initiate a security operation against them.
Acting Ukrainian President Oleksandr Turchynov on April 14 appealed to the UN to send peacekeeping troops to help in an operation to clear what he called "terrorists" from occupied areas in eastern Ukraine.
Besides the security problems in eastern Ukraine, the new government in Kyiv faces major economic challenges, including huge debts owed primarily to Russia for energy supplies.
The United States and European Union took some initial steps toward helping the new Ukrainian government on April 14.
The United States announced it had signed a $1 billion loan guarantee for Ukraine, while the European Union on April 14 announced a loan of nearly 1 billion euros ($1.4 billion) to Ukraine.
This comes in addition to 610 million euros of EU aid that has been approved but not yet disbursed.
Meanwhile, Putin has appointed Sergei Aksyonov to be the acting governor of Crimea until elections are held later this year.
Putin met with Aksyonov on April 14 at the Russian president's residence outside Moscow. He said that in accordance with legislation of the Russian Federation, it was necessary to name an acting governor.
Aksyonov said he would "justify the trust" Putin had shown him. He added there is much to do to complete the process of Crimea uniting with Russia.
Putin also confirmed that elections for governor and parliament would be held in Crimea on September 14. Aksyonov pledged that Crimea would be ready to hold the elections.
The elections are likely to increase tensions between Russia and the West. Western states in particular continue to say Russia's March annexation of Crimea from Ukraine is illegal.
And in the Black Sea, the U.S. Defense Department says a Russian fighter aircraft made repeated, low-altitude passes near a U.S. military ship.
Colonel Steve Warren, a Pentagon spokesman, condemned the actions of the Russian plane, which occurred on April 12, as "provocative and unprofessional."
He said the Russian Su-24 aircraft, known as a Fencer, made 12 passes at low altitude near the "USS Donald Cook," a destroyer that has been in the Black Sea since April 10.
He said the Russian plane appeared to be unarmed.
The U.S. ship, which carries helicopters, is now in a Romanian port.
With reporting by Reuters, AFP, AP, dpa, ITAR-TASS, and Interfax