WASHINGTON -- NATO foreign ministers met Thursday to celebrate the alliance's 70th anniversary amid rifts between members over security and trade issues.

Secretary of State Mike Pompeo opened the meeting with an appeal for unity to confront "great power" challenges from Russia, China and Iran. He hailed NATO's deterrence of the Soviet Union during the Cold War and said the alliance was well positioned to move forward as it confronts new and evolving challenges.

But a deepening dispute with Turkey over its planned purchase of a Russian air defence system and U.S. demands for allies, particularly Germany, to boost their defence spending threatened to overshadow the proceedings even as the ministers agreed on action to confront Russia's increasing assertiveness.

The ministers agreed on a package of measures to assist alliance aspirants Georgia and Ukraine, notably in the Black Sea with increased maritime co-operation, patrols and port visits, according to NATO chief Jens Stoltenburg. And, they renewed demands for Russia to end its annexation of Crimea and release Ukrainian sailors and ships it seized in a confrontation last year in the Sea of Azov.

"We have rightly sought peace through strength here at NATO," Pompeo said. "We must continue to do so, especially in this new era of great power competition from Russia, from China, and the Islamic Republic of Iran."

Addressing those challenges and others such as terrorism, cybercrime, uncontrolled migration, threats to energy security and new technologies will require enhanced resources, he said.

Pompeo said every NATO member had an obligation to explain to its citizens the need to increase their defence budgets and rejected what he called "tired arguments" about public opposition to such spending.

"These are real challenges, to be sure, and now is not the time to repeat tired excuses that our citizens don't support increased defence spending or security spending," he said.

Pompeo said convincing citizens "of the importance, the relevance, the intrinsic centrality of this institution" is "a key step in confronting these threats head on."

Stoltenberg, In an address to Congress on Wednesday and again on Thursday, acknowledged serious divisions within the alliance and called for bigger defence budgets to cope with global challenges such as Russian assertiveness, the core reason NATO was created in Washington 70 years ago this week.

"This is about all allies having to make good on the pledge they made," he said, stressing that additional resources were needed to cope with "a more challenging and demanding security environment."

President Donald Trump has questioned the value of the alliance and suggested that some members are freeloaders.

Pompeo did not address the spat with Turkey in his remarks, but on Wednesday in a meeting with Turkish Foreign Minister Mevlut Cavusoglu he had made clear Washington's displeasure with Ankara for insisting it would go ahead with plans to buy Russia's advanced S-400 system instead of the American Patriot system.

The Trump administration is threatening to stop delivery to Turkey of the newest U.S. fighter jet, the F-35, if the purchase is completed. Vice-President Mike Pence had delivered the same warning, noting that the U.S. and other NATO members had grave concerns about the S-400 as it is not interoperable with alliance systems.

"Turkey must choose," Pence said Wednesday. "Does it want to remain a critical partner in the most successful military alliance in the history of the world or does it want to risk the security of that partnership by making reckless decisions that undermine our alliance?"

Cavusoglu and other Turkish officials say they are proceeding with the deal because the need is urgent and the Patriots could not be delivered for another decade. They have also said the S-400 is not intended to work with NATO systems and will be a purely stand-alone defensive mechanism.

Stoltenburg acknowledged that the matter was a point of severe contention between the U.S. and Turkey and expressed hope that a resolution was possible. "We fully realize that this is now a challenge," he said.

Meanwhile, Canada's Foreign Minister Chrystia Freeland took the opportunity of the NATO meeting to register Ottawa's displeasure with being labeled a potential national security threat by the U.S. in relation to steel production. She called the designation, which has led to the imposition of tariffs on Canadian steel, "absurd" and pointed to her presence at the NATO meeting as proof that Canada is not a threat to the U.S.

"We really think this is groundless," she told reporters.