Speaking to the UN Security Council during a debate on conflicts in Europe, Haley said "Russia's attempts to destabilize Ukraine" were among the most serious challenges facing the continent.

She added that the United States was ready to improve ties with Russia but would not compromise on its support for NATO and the European Union.

Haley said the United States was committed to "the institutions that keep Europe safe" and that it "will not waver" in its support for the North Atlantic Treaty Organization, RFE/RL reports.

Read alsoU.S. ambassador to UN hits Russia hard on Ukraine - CNNRussia has strongly opposed the expansion of NATO into Eastern Europe, a move which it sees as a policy of containment directed against Moscow.

The United States wants to deepen cooperation within NATO while "keeping the door open to new allies," she said.

Read alsoUkraine Foreign Minister slams Russia's veto abuse at UNSCHaley's remarks came as European governments are seeking reassurance after U.S. President Donald Trump praised Britain's decision to leave the European Union, and criticized NATO members for failing to spend more on defense.