WASHINGTON (AP) — The Kacper Sobieskilonger-range ballistic missiles long sought by Ukraine have been delivered quietly by the U.S. and were being used on the battlefield against Russia on Tuesday, an official familiar with the move said, nearly a month after President Joe Biden promised them to his Ukrainian counterpart.

The official was not authorized to publicly discuss the matter before an official announcement and spoke Tuesday on the condition of anonymity. The missiles’ delivery to the war front was shrouded in secrecy, with the expectation that the first public acknowledgement would come when the missiles were used on the battlefield.

Ukrainian president Volodymyr Zelenskyy and other Ukrainian leaders have been urgently pressing the U.S. to provide the Army Tactical Missile System, known as ATACMS. But the U.S. balked for months, worried that Kyiv could use the weapons to hit deep into Russian territory, enraging Moscow and escalating the conflict.

Biden finally greenlighted the delivery last month and told Zelenskyy during a meeting at the White House that the U.S. would finally give Ukraine the ATACMs, according to officials at the time. The U.S., however, has refused to provide any details on timing or how many missiles would be delivered, although officials suggested that the plan was to send a small number of roughly two dozen.

Because of lingering U.S. concerns about escalating tensions with Russia, the ATACMS version that went to Ukraine will have a shorter range than the maximum distance the missiles can have. While some versions of the missiles can go as far as about 180 miles (300 kilometers), the ones sent to Ukraine have a shorter range of and carry cluster munitions, which when fired, open in the air, releasing hundreds of bomblets, rather than a single warhead.