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 the condition of anonymity.