Gen. Mark Milley, chairman of the U.S. Joint Chiefs of Staff, said Ukraine was “making steady progress,” but warned that the war would go on for some time.

“This is a very difficult fight,” he said. “It’s a very violent fight, and it will likely take a considerable amount of time at a high cost.”

But he praised the morale and flexibility of Ukrainian troops, contrasting them with Russian forces, saying: “Their leadership is not necessarily coherent, their troops’ morale is not high, and they’re sitting in defensive positions and many of them don’t even know why they’re there.”

The U.S. defense secretary, Lloyd J. Austin III, also praised Ukrainian resolve and the leadership of Denmark and the Netherlands, which are organizing training for Ukrainian pilots to fly American-made F-16s. Mr. Austin added, however, that it will be months before the pilots are ready.

Last month, the Biden administration dropped its objections to providing Kyiv with the F-16s, the advanced warplanes it had long sought.

“Ukraine’s fight is a marathon and not a sprint,” Mr. Austin said. “So we will continue to provide Ukraine with the urgent capabilities that it needs to meet this moment, as well as what it needs to keep itself secure for the long term from Russian aggression. And make no mistake. We will stand with Ukraine for the long haul.”

The allies are also working on supplying spare parts, ammunition and other equipment to keep Western-made Leopard tanks provided to Ukraine — already deployed as part of the counteroffensive — up and running, Mr. Austin said.

The announcement of the latest supply of air-defense missiles will help Kyiv fend off Russia’s “brazen missile and drone attacks against Ukrainian cities,” the four allies said in a joint statement.

Mr. Austin was in Brussels for a two-day meeting of NATO defense ministers, intended to prepare for the alliance’s yearly summit meeting in Vilnius, Lithuania, next month.

The NATO defense ministers plan to discuss a multiyear package of support and security arrangements for Ukraine, as well as efforts to bolster defense production across the alliance to help address Ukraine’s ammunition needs.

The ministers are also expected to review new regional plans to defend NATO territory that the alliance’s secretary general, Jens Stoltenberg, claimed could put “well over 300,000 troops at high readiness,” as part of the alliance’s efforts to revitalize its military strategy after Russia’s full-scale invasion of Ukraine.

The meeting comes amid disputes over Sweden’s pending membership bid. Turkey, which has so far blocked Sweden’s membership, has indicated little movement so far in its refusal. Still, Mr. Stoltenberg expressed optimism that Sweden, having met Turkey’s demands to toughen up its terrorism laws, will become a member by the meeting in Vilnius or soon afterward.

The summit will also include discussions of a successor to Mr. Stoltenberg, but that process is complicated and Mr. Stoltenberg may be asked to stay on for several more months. A leading possibility, the Danish prime minister, Mette Frederiksen, said on Thursday that she was no longer a candidate. She said that she would back Mr. Stoltenberg if he was willing to extend his mandate.

Britain’s defense secretary, Ben Wallace, has made it clear he would like the job, but France and other allies are insisting that a new secretary general come from a country of the European Union.