Nato foreign ministers on Thursday approved measures aimed at countering Russia in the Black Sea region.
In a meeting in Washington to mark the alliance's 70th anniversary, ministers agreed to provide Georgia and Ukraine with increased naval co-operation, patrols and port visits.
Both countries have faced Russian aggression and aspire to join the alliance.
The Nato ministers also renewed demands for Russia to end its annexation of Crimea, release Ukrainian sailors and ships it seized in a confrontation last year in the Sea of Azov, and respect the Intermediate-Range Nuclear Forces Treaty.
The US announced it would withdraw from the 1987 treaty in August unless Russia returned to compliance.
But a dispute between the US and Turkey over Ankara's planned purchase of a Russian air defence system, US demands for allies to boost defence spending, and a row with Canada over tariffs dulled the commemoration ceremony at the State Department.
Secretary of State Mike Pompeo appealed for the allies to stand together to confront "great power" challenges from Russia, China and Iran.
Mr Pompeo hailed Nato's deterrence of the Soviet Union during the Cold War and said that the alliance was well placed to confront new and evolving challenges.
"We have rightly sought peace through strength here at Nato," he 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, cyber crime, uncontrolled migration, threats to energy security and new technology will need better resources, Mr Pompeo said.
He also renewed a warning that the US might have to curtail intelligence co-operation with countries that rely on Chinese communications technology.
Mr 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 excuses" about public opposition to such spending.
"We're very hopeful that they will get it right, that they will understand that it is important for our collective defence," he said later, referring specifically to Germany.
Nato Secretary General Jens Stoltenberg, in an address to Congress on Wednesday and again on Thursday, acknowledged serious divisions within the alliance and called for bigger defence budgets.
US President Donald Trump has questioned the value of the alliance and criticised some members for not spending enough.
Mr Pompeo did not address the dispute with Turkey in his remarks.
But in a Wednesday meeting with Turkish Foreign Minister Mevlut Cavusoglu he made it clear that Washington was displeased with Ankara for insisting it would buy Russia's advanced S-400 system instead of the American Patriot system.
As we celebrate #NATO70, we honor an Alliance that has been and still is the bulwark of international peace and security. The U.S. affirms our unconditional support for @NATO and Article 5. Together, we are #AlliedStrong. pic.twitter.com/H6KwSLCH3q
— Secretary Pompeo (@SecPompeo) April 4, 2019
Turkey's Foreign Ministry disputed the State Department's account of the meeting, which said Mr Pompeo warned of "potentially devastating consequences" of unilateral Turkish military action in northern Syria and consequences of buying the S-400.
The ministry said the account failed to accurately reflect the content of the discussion and added that "our alliance naturally requires that such statements are prepared with greater care".
Asked about the Turkish complaint, Mr Pompeo said he had re-read the account and it was "spot on".
''I stand by every word of it," he said, adding that his meeting with Mr Cavusoglu had been good and that the Turks were well aware of the US position.
The Trump administration is threatening to stop delivery to Turkey of the newest US fighter jet, the F-35, if the S-400 purchase is completed.
Vice President Mike Pence delivered the same warning, noting that the US and other Nato members had grave concerns about the S-400 because it is not interoperable with alliance systems.
"Turkey must choose," Mr Pence said on 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?"
Mr 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 standalone defence.
Mr Stoltenberg acknowledged that the matter was a point of severe contention between the US and Turkey and expressed hope that a resolution was possible.
Meanwhile, Canada's Foreign Minister Chrystia Freeland took the opportunity to tell of the country's displeasure at being called a potential national security threat by the US over steel production.
Ms Freeland 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 US.
"We really think this is groundless," she said after the meeting.
