Ukrainian troops will begin training on American M1 Abrams tanks in Germany in the next few weeks, U.S. defense officials say, in what would be a major step in arming Kyiv as it seeks to seize back territory from Russia.

Defense Secretary Lloyd Austin announced the timeline on Friday during a meeting with allies at Ramstein Air Base. Defense officials said that about 31 tanks were expected to arrive in Germany to begin a training program for Ukrainian troops that is expected to take 10 weeks. Combat-ready tanks could reach the battlefields in Ukraine by the fall, said the officials, who spoke on the condition of anonymity to discuss security matters.

But the United States stood firm in its refusal to supply Ukraine with F-16 fighter jets. Speaking at a news conference after the meeting, Gen. Mark Milley, the chairman of the Joint Chiefs of Staff, said that Ukraine’s air-defense system had worked effectively for more than a year and kept Russian warplanes "cautious” for fear of being shot down.