WASHINGTON (Reuters) - The head of the North Atlantic Treaty Organization warned the U.S. Congress on Wednesday of the threat posed by “a more assertive Russia,” including a massive military buildup, threats to sovereign states, the use of nerve agents and cyberattacks.

“We must overcome our differences now because we will need our alliance even more in the future. We face unprecedented challenges - challenges no one nation can face alone,” NATO Secretary General Jens Stoltenberg said.

Saying “time is running out,” Stoltenberg also called on Russia to return to compliance with the Intermediate-range Nuclear Forces Treaty, from which U.S. President Donald Trump plans to withdraw the United States this summer.

“NATO has no intention of deploying land-based nuclear missiles in Europe,” Stoltenberg said. “But NATO will always take the necessary steps to provide credible and effective deterrence.”

Stoltenberg used his speech to give an impassioned defense of the 70-year-old Western alliance, a partnership he called “the most successful alliance in history,” which has often been derided by Trump since he took office in 2017.

Members of Congress, who greeted Stoltenberg with repeated cheers and standing ovations, said they viewed his address to the joint meeting of the House of Representatives and Senate as a chance to reaffirm the American commitment to the NATO alliance.

Stoltenberg was the first leader of an international organization and the first Norwegian to be accorded the rare honor of such an address.

‘GOOD FOR THE UNITED STATES’

“NATO has been good for Europe, but NATO has also been good for the United States,” Stoltenberg said. The invitation to Stoltenberg to speak as the alliance celebrates its 70th anniversary in Washington was widely seen as a way to send a message to Trump about strong bipartisan support for NATO.

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“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,” Stoltenberg added. “And through NATO the United States has more friends and allies than any other power. This has made the United States stronger, safer and more secure.”

The Senate is due to vote later this year on whether North Macedonia can become NATO’s 30th member.

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Trump has ruffled feathers among European allies by repeatedly saying NATO nations need to pay more for their militaries and ease the burden on the United States.

Earlier this year, before inviting Stoltenberg to Congress, House Speaker Nancy Pelosi led a delegation of her fellow Democrats to Brussels, where they sought to reassure European allies that differences over Trump’s policies were mere “family squabbles” and that transatlantic ties remained strong.

Members of Congress have also introduced legislation expressing support for NATO or seeking to keep any president from withdrawing from the alliance without lawmakers’ approval.

Stoltenberg met on Tuesday with Trump, who said his pressure on NATO nations to pay more for their defense is leading to tens of billions of dollars more in contributions, but the allies may need to boost their budgets even more.

Stoltenberg said NATO member countries are adding billions to their defense spending - $41 billion in the last two years. He expects that figure to rise to $100 billion next year.

“This is making NATO stronger,” he said. “This is good for Europe, and it is good for America.”