Denmark and the Netherlands have started transferring the planes, with Belgium and Norway committing to provide further aircraft, a White House statement said.

"As we speak, the transfer of F-16 jets is underway, coming from Denmark, coming from the Netherlands," Secretary of State Antony Blinken said at a NATO 75th anniversary summit in Washington.

"Those jets will be flying in the skies of Ukraine this summer to make sure that Ukraine can continue to effectively defend itself against the Russian aggression," he said.

Ukraine has long sought advanced Western aircraft and President Joe Biden last August gave the green light to transfer American-made F-16s to the country, despite earlier concerns about how long it would take for Kyiv to train on the planes.

Blinken said the transfer of the jets should be a signal to Russian President Vladimir Putin.

"It concentrates Vladimir Putin's mind on the fact that he will not outlast Ukraine, he will not outlast us and, if he persists, the damage that will continue to be done to Russia and its interests will only deepen," Blinken said.

"The quickest way to get to peace is through a strong Ukraine," the top American diplomat added.