Washington's readiness to provide support to its NATO allies on the eastern flank using Article 5 of the NATO Charter amid Russia's buildup of troops on the border with Ukraine was announced by President Joe Biden's National Security Advisor Jake Sullivan.

"Mr. Sullivan reiterated U.S. support for the North Atlantic Treaty Organization's Article 5 and underscored the United States' readiness to support our Eastern Flank Allies as they defend their countries," the White House said in a phone conversation between Sullivan and Poland's National Security Bureau Chief Pawel Soloch.

The White House said the sides "exchanged views on Russia's military build-up near Ukraine's border and affirmed the importance of Allied unity in diplomatic and deterrence efforts."

Article 5 of the NATO Charter provides that an armed attack on one or more member countries of the alliance will be considered as an attack on all members of the North Atlantic Treaty.