Continuing his diplomatic stand-off with Vladimir Putin, U.S. President Joe Biden said Russia is making a ''big mistake'' by suspending a key nuclear arms treaty.

Biden made the brief comment to reporters as he entered the presidential palace in Warsaw where he is meeting with leaders from nations on the eastern edge of the NATO alliance. It is the latest in a series of verbal jockeying between Russia and the U.S. as Biden wraps up a three-day visit to the region.

Putin announced Tuesday he is suspending Moscow’s participation in New START, a strategic nuclear arms reduction treaty, the last remaining nuclear arms reduction deal between the U.S. and Russia. It limits each side to 1,550 long-range nuclear warheads.

Biden is ending his three-day trip to Ukraine and Poland Wednesday with a final emphasis on the strength of the NATO alliance, which has stood up to  Putin.

Addressing the world in a speech at Warsaw's Royal Castle Tuesday, Biden said the U.S. commitment to the alliance and its mutual defense pact is "rock solid."

“And every member of NATO knows it.  And Russia knows it as well,” Biden said. "It's a sacred oath, a scared oath to defend every inch of NATO territory.”

The latest

• Trip overview: After making a surprise visit to war-torn Ukraine Monday, Biden traveled to Poland where he praised the strength of Ukraine and the international coalition backing the resistance.

• Bucharest Nine: Biden is meetiings Wednesday with leaders of Bulgaria, the Czech Republic, Estonia, Hungary, Latvia, Lithuania, Poland, Romania and Slovakia. The countries are known as the Bucharest Nine, a group of eastern NATO allies formed in 2015 in response to Russia’s annexation of Crimea.

• Frontlines: The Bucharest Nine are "on the frontlines of our collective defense right now," White House spokesman John Kirby said when previewing Biden's meeting.. The nations have advocated for an increased NATO presence and deterrence measures in the region.

• NATO anniversary: Biden announced Tuesday the U.S. will host a NATO summit next year to mark the 75th anniversary of what he called the "strongest defensive alliance in the history of the world."

Why it matters

Russia has tried to keep NATO from its borders. In the lead-up to its invasion of Ukraine, Putin demanded that Ukraine never be allowed to join the alliance. He also wanted to keep NATO missiles from being in striking distance and stop the alliance from deploying forces in former Soviet bloc countries that joined NATO after 1997.

Instead, the alliance has grown stronger. Finland and Sweden are in the process of joining. NATO has bolstered its troop presence in Europe. And NATO countries have provided military and other support to Ukraine.

"NATO will not be divided, and we will not tire," Biden said Tuesday.

But the administration and its allies have also tried to keep the fight against Russia from spilling into a NATO country to avoid triggering the mutual defense pact.

Looking ahead

While praising allies for standing together, Biden is emphasizing that the fight is far from over. There will be "hard and very bitter days, victories and tragedies," he said Tuesday.

Biden also encouraged other nations to look beyond the immediate challenge of the war in Ukraine and also work together in "lifting up the lives of people everywhere."

"The democracies of the world have to deliver it for our people," he said.

How many times has Article 5 been invoked?

The North Atlantic Treaty Organization was created in 1949 as a system of collective defense against the Soviet Union.

Its basic principle of mutual defense is the Article 5 provision that requires member states to come to the aid of their allies in the event of an attack.

The article has been invoked only once, when the U.S. called on the alliance after the Sept. 11, 2001 terror attacks.

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This article originally appeared on USA TODAY: Biden criticizes Putin's suspension of New START nuclear arms treaty