And the U.S. official did not rule out the possibility of the Biden administration issuing re-export licenses to European militaries, enabling them to transfer their F-16s to Ukraine. Later Tuesday, after Britain and the Netherlands announced their so-called “fighter coalition,” Secretary of State Antony J. Blinken and the Dutch foreign minister, Wopke Hoekstra, spoke by phone to discuss Ukraine and other issues.

Mr. Hoekstra said on Wednesday that “we haven’t reached a solution yet” in what another senior European diplomat described as a slow-moving and difficult discussion.

“When we are ready to cross that bridge and are ready to communicate this, we will,” Mr. Hoekstra said.

The Netherlands is one of four European countries that the senior Ukrainian official said have quietly signaled they are ready to send F-16s to Kyiv. Its fleet, along with those of Denmark and Belgium, could provide at least 125 combat-ready F-16s, according to the International Institute for Strategic Studies, a British think tank that assesses military stockpiles globally. Norway, which retired its unspecified number of F-16s last year in a switch to the more advanced F-35 Joint Strike Fighter, is also ready to contribute, the Ukrainian official said.

Kyiv is asking — for now, at least — for only between 24 and 36, the official said.

Earlier this week, the British prime minister, Rishi Sunak, said the United Kingdom would begin training Ukrainian pilots, starting this summer, as part of a plan “with other countries on providing F-16 jets.” His announcement, wrapped in a new package of military aid, came during a visit to London by President Volodymyr Zelensky of Ukraine.