NATO said Saturday its top policymakers would meet after Ukraine urged the U.S.-led alliance to step in to protect its "territorial integrity" from incursions by Russia.
Foreign Minister Sergei Deshchiritsya said he made the request after discussions with U.S. and European Union officials. He specifically mentioned the threat to "nuclear facilities on Ukrainian territory."
The North Atlantic Council, NATO's top policy committee, said it would convene a meeting Sunday afternoon, followed by a meeting of NATO's Ukraine Commission, which coordinates the alliance's relations with Kiev.
NATO's potential intercession was welcomed by pro-Ukrainian activists and Sen. John McCain of Arizona, a leading Republican spokesman on foreign affairs.
"NATO ships in the Black Sea should organize a non-invasive blockade to stop Russian ships exiting," said Ilona Doerfler, a young woman from Kiev who was part of a demonstration outside the White House on Saturday afternoon.
"It is not about the Ukraine," she told NBC News. "It's about the integrity of promises to respect territorial sovereignty."
McCain, meanwhile, urged President Barack Obama to take a harder line on Ukraine, specifically calling on the president to "rally our European and NATO allies to make clear what costs Russia will face for its aggression and to impose those consequences without further delay."
Kristen Welker of NBC News contributed to this report.