By Mike Stone and Humeyra Pamuk

WASHINGTON (Reuters) -President Joe Biden told G7 leaders on Friday that Washington supports joint allied training programs for Ukrainian pilots on F-16 fighter jets, senior U.S. officials said, a significant endorsement as Kyiv seeks to boost its air power against Russia.

Training on the U.S.-made jets will take place in Europe and will require months to complete, one of the officials said. U.S. officials have estimated the most expeditious time needed for training and delivery of F-16s at 18 months.

"As the training takes place over the coming months, our coalition of countries participating in this effort will decide when to actually provide jets, how many we will provide, and who will provide them," the official said.

Although the official did not say which countries would participate, British Prime Minister Rishi Sunak said the UK would work with the Netherlands, Belgium and Denmark "to get Ukraine the combat air capability it needs."

Denmark's defense ministry said it was ready to support the training.

The U.S. hopes to begin the training, which will be conducted on fourth-generation fighters including F-16s, in the coming weeks, the official added.

Ukraine is seeking the jets, made by Lockheed Martin, as it presses on with its biggest advance for months against invading Russian troops before a planned counteroffensive.

Ukraine, which does not possess any Western-designed jets, says the F-16s are far more effective than the Soviet-era fighters it still uses. Poland and Slovakia have handed over 27 MiG-29s to Ukraine.

Western governments have been wary of leaving their own countries undefended by giving away too much equipment. They have also avoided sending anything that could strike deep into Russian territory and give Moscow a reason to attack.

Ukraine's President Volodymyr Zelenskiy received pledges this week from Britain and the Netherlands to help build a "jets coalition", although leaders from both countries stopped short of saying they will send planes.

On Friday, Zelenskiy welcomed the news. "This will greatly enhance our army in the sky. I count on discussing the practical implementation of this decision at the #G7 summit in Hiroshima," he said on Twitter.

(Additional reporting by Rami Ayyub, Phil Stewart, Mike Holden in London and David Ljunggren; editing by Doina Chiacu, Bill Berkrot and Grant McCool)