In a long-awaited move, the United States will allow allies to send American-made F-16 fighter jets to Kyiv once Ukrainian pilots are trained to operate them, a U.S. official confirmed on Thursday. However, the requirement that Ukraine’s pilots be fully trained means that the approvals will not come for months.

Ukraine’s counteroffensive, which began two months ago, has a chance of prevailing without the fighter jets, experts say, but it is likely to be far more difficult.

The time frame for the U.S. approvals is unlikely to surprise Ukraine. A spokesman for Ukraine’s Air Force, Yuriy Ihnat, said on Wednesday that it had become clear the country would not be able to operate or receive F-16 fighter jets this coming fall or winter, confirming that the planes will not play a role in the counteroffensive.

“We had high hopes for this aircraft,” he said.

The U.S. decision had been anticipated since May, when President Biden eased his resistance to NATO allies’ efforts to train Ukrainian pilots on F-16s and provide the jets to Ukraine. The official who confirmed the U.S. shift was not authorized to publicly discuss the agreement and spoke on condition of anonymity.

Some of the pilot training may now occur in the United States, a Pentagon spokesman said on Thursday.

“The U.S. is prepared to support the training effort in coordination with the coalition, and is willing to host training for Ukrainian pilots within the U.S. if the capacity is reached in Europe,” Brig. Gen. Patrick S. Ryder, the Pentagon press secretary, said in a statement.

Ukraine has Soviet-era MIG and Sukhoi fighter jets, but has long argued that F-16s could enable it to achieve air superiority, something neither side has decisively attained since Russia launched its full-scale invasion in February 2022.

The U.S. transfer approvals, when they come, are expected to go to Denmark and the Netherlands, which are leading a coalition to train the pilots. A March assessment by the U.S. Air Force suggested the most rapid time frame for full training would be four to six months; other estimates are longer. And time-consuming training on how to maintain the jets will also be necessary.

American officials have said that Ukraine has identified only eight combat pilots who speak English well enough to start training. That is fewer than necessary for a single squadron. About 20 others are being sent to Britain this month to learn English.

The F-16, or Fighting Falcon, first flown in 1976, is used by militaries in dozens of countries for air-to-air combat and air-to-ground strikes.

The F-16 fighter jets are built by the American defense contractor Lockheed Martin and are manufactured in Belgium, Denmark, the Netherlands and Norway. All four countries have signaled their willingness to transfer the planes to Kyiv, according to a senior Ukrainian official.

In May, before Mr. Biden agreed to allow the Ukrainian pilots to be trained on F-16s, the leaders of Britain and the Netherlands announced an international coalition to provide Ukraine with the fighter jets and the training to fly them. Prime Minister Rishi Sunak of Britain said then that the training would begin this summer.

In July, Denmark’s acting defense minister, Troels Poulsen, told reporters that the country hoped to see “results” from the training early next year.