WASHINGTON - US Vice President Joe Biden on Tuesday reaffirmed steadfast American commitment to NATO's collective defense amid the ongoing crisis in Ukraine, the White House said.

In a phone call with Slovak President Andrej Kiska, Biden noted the strong bilateral relationship between Slovakia and the United States, and welcomed Kiska's support for strengthening the transatlantic relationship and for Slovakia's contributions to NATO, the White House said in a statement.

The two leaders also discussed the need to continue to impose sanctions on Russia for its "aggressive acts" in Ukraine, said the a statement.

In the telephone talk, Kiska noted that the reverse flow of gas began Tuesday between Slovakia and Ukraine, which can help Ukraine meet some of its energy needs. Gas flows from Slovakia to Ukraine will be delivered through the Vojany-Uzhgorod pipeline at a rate of 10 billion cubic meters per year.

In April, Ukrtransgaz, the transport arm of Naftogaz, the national oil and gas company of Ukraine, and Slovak pipeline operator Eustream signed an agreement on gas deliveries, which is aimed at diversifying energy supplies to Ukraine.

On Tuesday, US President Barack Obama left for Estonia for a visit aimed at reaffirming US commitment to its Baltic allies amid the crisis in Ukraine. After visiting Estonia, he will proceed to Britain to attend a NATO summit scheduled to be held in Wales this week.