Closing two days of talks in Washington, foreign ministers from the 29-nation alliance said in a joint statement that they face "an unpredictable and challenging security situation" that includes "a more aggressive Russia."

NATO Secretary General Jens Stoltenberg said the alliance approved new surveillance measures and naval exercises in the Black Sea to back Ukraine and Georgia, aspiring members of NATO that are facing Russian-backed separatist forces.

Stoltenberg urged Russia to return three Ukrainian navy vessels and its sailors which it seized in November near Crimea -- the peninsula that Moscow annexed in 2014, in a move not recognized internationally.

"We are stepping up our efforts in the Black Sea region," Stoltenberg told reporters.

US Secretary of State Mike Pompeo said that NATO members agreed to study strategies to face non-traditional warfare from Russia, which is accused of meddling in a series of Western elections.

"Vladimir Putin harbors dark dreams of imperialism. This is evident from his invasions of Georgia and Ukraine, his meddling in Syria and now in Venezuela. He wants to split our alliance and weaken our democratic resolve," Pompeo said.

- 'The Eastern threat' -

Russian President Vladimir Putin counters that NATO has posed a threat by its eastward expansion into the former Soviet bloc and accuses the United States of plotting a coup with its bid to oust Venezuela's leftist leader Nicolas Maduro.

Russia criticized the Black Sea plan and, in a statement for the NATO anniversary, voiced regret that the alliance "has no intention to renounce its political and military confrontation with Russia."

"It is time to stop bringing back the 'Eastern threat.' In the interest of peace, the world needs a de-escalation of military and political tensions," the foreign ministry said.

Stoltenberg, asked by a reporter if NATO was provoking Russia by considering membership for Ukraine and Georgia, said that "big powers" cannot dictate terms to others.

"As soon as you accept that that's a provocation against Russia, then you accept that Russia has the right to decide what neighbors can do," he said.

The renewed worries about Russia come after a decade in which NATO's focus has shifted well beyond Europe, with the alliance engaged for nearly 18 years in Afghanistan.

Pompeo briefed the allies on US efforts to broker a solution with Taliban militants to end the war triggered by the September 11, 2001 attacks.

- Internal divisions -

He said that NATO needed to confront "Russian aggression" but also "emerging threats" including the rise of China as well as Iran.

The United States has enjoyed less support over those two issues, with all of its NATO allies backing an international accord on ending Iran's nuclear program from which President Donald Trump withdrew.

Pompeo has also been seeking with limited success to persuade US allies not to choose Chinese telecom giant Huawei for their fifth-generation networks, warning that the company poses risks to security and personal privacy.

The Trump administration has been livid at Germany, Europe's largest economy, for its budgetary plans that do not come close to the NATO goal of spending two percent on defense spending.

"Now is not the time to repeat tired excuses that our citizens don't support increased defense or security spending. Each nation has the duty to make the case to our people," Pompeo said.

NATO allies have also seen rising friction with Turkey, which has been clamping down on dissent and has defiantly insisted on buying a major missile defense system from Russia, even after the United States suspended its participation in the F-35 fighter-jet program.

But the NATO meeting witnessed the end to one long-running standoff with the participation of the foreign minister of North Macedonia.

The former Yugoslav republic had been in limbo for years, with Greece objecting to the name of Macedonia, which is historically tied to Hellenic rather than Slavic civilization.

Greece's left-wing prime minister, Alexis Tsipras, brokered a deal to welcome the country after it changed its name to North Macedonia and it is expected to enter NATO as its 30th member later this year.

Greek Foreign Minister Georgios Katrougalos said the deal can offer a "blueprint" for resolving conflicts as leaders need not see "history as a prison."