NATO's chief says the alliance faces threats from a "more assertive Russia," but also that the alliance is not seeking to isolate Moscow.
Jens Stoltenberg made the comments on April 3 in a speech before a joint meeting of Congress -- the first time a NATO secretary-general has addressed the U.S. legislative body.
In his speech, Stoltenberg criticized what he called a "more assertive Russia," and he called on Moscow to return to compliance with key Cold War-era arms-control treaties.
"We do not want to isolate Russia, we strive for effective relations with Russia," he said.
The treaty in question -- the 1987 Intermediate-Range Nuclear Forces Treaty -- is on the verge of collapse, after Washington earlier this year declared it was suspending compliance. The move followed years of U.S. accusations that Moscow had violated the treaty by testing, and later deploying, a missile that was prohibited under its restrictions. Moscow has denied the missile in question violates the treaty.
"NATO has no intention of deploying land-based nuclear missiles in Europe. But NATO will always take the necessary steps to provide credible and effective deterrence," Stoltenberg said.
"We do not want a new arms race. We do not want a new Cold War. But we must not be naive. An agreement that is only respected by one side will not keep us safe," he said.
WATCH: RFE/RL takes a look at NATO's Cold War origins, its first battles in the Balkans, and its latter embrace of former foes.
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Stoltenberg's visit comes as foreign ministers from the alliance's 29 member countries are gathering in Washington to mark its 70th anniversary and discuss security threats, including Russia and Afghanistan.
The gathering also comes amid persistent tensions with President Donald Trump about the how the alliance is funded.
Trump has regularly complained that members are not sharing the alliance's financial burden fairly, something he raised again in a White House meeting with Stoltenberg a day earlier.
"We are protecting countries that have taken advantage of the United States," Trump said. "The United States pays for a disproportionate share of NATO," he added. "We just want fairness."
Stoltenberg appeared to support that argument, which is shared by some Republicans in Congress.
"NATO is a strong alliance. But to remain a strong alliance, NATO must be a fair alliance," he said. "In an ideal world, we would not need to spend any money on defense. But we do not live in an ideal world."
"Allies must spend more on defense. This has been the clear message from President Trump. And this message is having a real impact," he said.
The United States is by far the largest contributor of funding to NATO, followed by Germany, Britain, and France.
Spending by the 29 NATO countries, which dropped after the end of the Cold War, has in fact been on the rise since 2014 -- before Trump took office.
After Russia annexed Ukraine's Crimean Peninsula in 2014, allies agreed to boost defense spending and "move toward" spending 2 percent of their GDP on defense by 2024. The United States spends 3.4 percent of its GDP on defense.
Trump has singled out Germany, which recently announced plans to increase defense spending to 1.25 percent of its GDP by 2023 -- a revision of Chancellor Angela Merkel's pledge last year to hit 1.5 percent by 2024.
Trump has also openly questioned the most important aspect of the alliance: that an attack on one member is considered an attack on all members.
In his speech, Stoltenberg hailed what he said was the U.S. commitment to the alliance, saying it was indicative of the warm relations that Washington has historically had with European partners.
"We have experienced an unprecedented period of peace. So the NATO alliance is not only the longest-lasting alliance in history, it is the most successful alliance in history," he said.
"The strength of a nation is not only measured by the size of its economy or the number of its soldiers but also by the number of its friends," he said.
NATO foreign ministers are scheduled to hold meetings at the State Department on April 4, with a first session focusing on Russia.
They are expected to endorse a set of measures in the Black Sea to improve NATO's defenses in the region.
The situation in Afghanistan as well as the demise of a landmark arms-control treaty that the United States and Russia signed in the last years of the Cold War, and NATO-member Turkey's decision to buy a Russian surface-to-air missile system, will also be on the agenda.
The Russian system is not compatible with NATO systems and is considered a threat to U.S. F-35 aircraft.