President Joe Biden said Thursday the United States must uphold its "sacred commitment" to NATO as the alliance marked its 75th anniversary, after election rival Donald Trump undermined its collective defense guarantee.

"We must remember that the sacred commitment we make to our Allies -- to defend every inch of NATO territory -- makes us safer too," Biden said in a statement.

NATO allies have been increasingly unnerved by the prospect of a Trump return to the White House in November's US presidential election.

Trump recently said he'd encourage Russia to attack any members not spending enough on defense, and while in office he considered pulling out of the alliance over accusations that other states were not paying their way.

Biden meanwhile hailed NATO's unity over the past two years as the alliance stepped up military support for Ukraine in the face of Russia's "vicious invasion".

With the addition of Finland and Sweden, who both joined in the wake of the Ukraine war, the alliance was now "larger, stronger, and more determined than ever before," he said.

"As our adversaries have plotted to break our remarkable unity, our democracies have stood unwavering," added Biden, 81.

The United States will host NATO's 75th anniversary summit in Washington in July.