Washington has told Denmark and the Netherlands that they will be permitted to hand over their F-16 fighter jets to Ukraine when the country's pilots are trained to operate them, the US State Department said Friday.

Both Denmark and the Netherlands, which are leading the program to train Ukraine's pilots on the F-16, have been given "formal assurances" for a jet transfer, a State Department spokesman said.

"This way, Ukraine can take full advantage of its new capabilities as soon as the first set of pilots complete their training," the spokesman said.

The United States keeps tight restrictions on the resale or transfer of US-made military equipment by allies.

It remained unclear how long it would take before the first Ukrainian pilots are ready to fly F-16s.

Training by an 11-nation coalition was to begin this month, and officials have said they hoped for pilots to be ready by early 2024.

Ukrainian Minister of Defense Oleksii Reznikov hailed "great news from our friends in the United States".

Netherlands Defense Minister Kajsa Ollongren also welcomed the move.

"It allows us to follow through on the training of Ukrainian pilots," she said on social media.

However, "everything takes time," she said later, according to the ANP new agency.

The F-16 program responds to requests by Ukraine to replace the heavy losses incurred by its air force, which flies mostly Russian aircraft.

The US jet has better combat capabilities than those operated by Ukraine, but requires more training for pilots.

Kyiv has been pushing hard since last year for the US-made jets, but Washington only gave its nod in recent months.

In July, a senior Ukrainian official said they wanted dozens of the aircraft to back up offensive ground operations in the south, where Kyiv hopes to drive out Russian forces.

"We need 60 to 80 F-16 planes in order to close the skies well, especially in the area near the front," said Mykhailo Podolyak, a senior aide to President Volodymyr Zelensky.

While the delivery of fighter jets should help Ukraine in the long term, its army is unlikely to benefit from them quickly, even though its June counter-offensive has so far yielded limited results.