KYIV, Ukraine — NATO’s Secretary-General, Jens Stoltenberg, visited Ukraine on Thursday for the first time since Russia invaded more than a year ago, a highly symbolic trip that underscores the alliance’s commitment to helping Kyiv defend itself.

The Kremlin quickly warned that Ukraine must not be allowed to join NATO. Russia has given various and shifting justifications for going to war, but it has repeatedly pointed to the expansion of the military alliance toward its borders in recent years, including citing fears that Kyiv would be admitted.

Images published in local press showed Mr. Stoltenberg apparently paying tribute to fallen Ukrainian soldiers at Kyiv’s St. Michael’s Square.

The visit, just two days after President Putin himself went to Ukraine, holds important symbolism, but its exact purpose wasn’t immediately clear.

NATO has no official presence in Ukraine, but Mr. Stoltenberg has been the strong voice of the alliance throughout the war. 

He has been instrumental in garnering and coordinating support — including weapons, ammunition and training for Ukraine’s embattled troops — from the 31 countries that make up the organization.

NATO itself only provides nonlethal support — generators, medical equipment, tents, military uniforms and other supplies — to the government in Kyiv.

A procession of international leaders has made the journey to Kyiv over the last year and the former Norwegian prime minister is one of the last major Western figures to do so.

NATO, formed to counter the Soviet Union, has long feared being dragged into a wide war with nuclear-armed Russia, but as the West has moved from hesitantly providing helmets and uniforms to tanks, warplanes and advanced missile systems, high-level visits have become routine.

Mr. Stoltenberg had been to Kyiv before the war, but this is his first visit during the hostilities. NATO leaders said in 2008 that Ukraine would join the alliance one day, and Mr. Stoltenberg has repeated that promise throughout the course of the war.

A Kremlin spokesman, Dmitry Peskov, said that preventing Ukraine from joining NATO remains one of the goals of what Moscow calls its “special military operation.” 

Speaking in a conference call Thursday with reporters, Mr. Peskov said that Ukraine’s accession would pose a “serious, significant threat to our country, to our country’s security.”

Earlier this month, Finland joined the alliance, setting aside decades of neutrality in a historic realignment of Europe’s post-Cold War security landscape. While NATO says it poses no threat to Russia, the Nordic country’s accession dealt a major political blow to Mr. Putin.

Finland’s membership doubles Russia’s border with the world’s biggest security alliance. Neighboring Sweden is expected to join in coming months, too, possibly by the time President Biden and his NATO counterparts meet at the Lithuanian capital Vilnius in July.

The alliance has focused on bolstering defenses on its own territory to dissuade Mr. Putin from attacking any member country. Under NATO’s collective security guarantee, an attack on one member country is considered to be an attack on them all.

On Friday, Mr. Stoltenberg will attend a Ukraine Defense Contact Group meeting at Ramstein Air Base, Germany, with Defense Secretary Austin. It’s the main international forum for drumming up military support for the conflict-ravaged country.

Denmark and the Netherlands announced Thursday that they plan to provide Ukraine with at least 14 refurbished German-made Leopard 2 battle tanks, starting in early 2024.

The announcement comes on top of a previous pledge, by Denmark, the Netherlands and Germany, to supply at least 100 older Leopard 1 A5 tanks.