NATO Secretary General Jens Stoltenberg visited U.S. troops at Lask Airbase, Poland Monday.
In a press conference after the visit, Stoltenberg said Russian President Vladimir Putin has shattered the peace in Europe.
"The Russian assault is totally unacceptable. And, it is enabled by Belarus," he said.
"We welcome the leading role of Poland," Stoltenberg said, noting that Poland has opened its border to hundreds of thousands of refugees fleeing the conflict.
NATO will always stand together, he said, mentioning that fighter jets from the United States are now flying alongside the Polish Air Force, keeping the skies over NATO nations safe 24/7.
French troops are arriving in Romania, as well, Stoltenberg noted.
NATO's commitment to Article 5 is iron-clad, he said, referring to the alliance's collective security defense.
Later in the day, Stoltenberg and U.S. Air Force Gen. Tod D. Wolters, the commander of U.S. European Command and NATO's supreme allied commander for Europe, visited NATO's multinational battlegroup in Tapa, Estonia. They were joined by British Prime Minister Boris Johnson.
Wolters thanked Estonian Prime Minister Kaja Kallas for hosting U.S., U.K. and other NATO forces in her nation.
The general noted that rigorous military training has enabled U.S. forces to be confident, responsive, resilient, competent and capable in whatever tasks are asked of them.
"We thank you for what you are doing today," Wolters told the troops in Estonia. "You are protecting the sovereignty of our NATO nations and our territories and we're in the business of generating peace."
At a press conference with Johnson and Kallas, Stoltenberg said: "Our message to President Putin is: Stop the war. Pull out all your forces from Ukraine and engage in good faith in diplomatic efforts. The world stands with Ukraine."
He also stressed that allies will protect and defend every inch of NATO territory, and thanked Johnson for doubling the U.K. troop presence in the NATO battlegroup.