The first batch of U.S.-built F-16 fighter jets are already being transferred to Ukraine from Denmark and the Netherlands and will be flying over Ukrainian skies this summer, U.S. Secretary of State Antony Blinken said on Wednesday.

Blinken, speaking at an event on the sidelines of the NATO summit in Washington, said a robust package for Ukraine will be unveiled over the next couple of days that will build a clear and strong bridge for Ukraine's NATO membership.

"I'm also pleased to announce that as we speak, the transfer of F-16 jets is underway, coming from Denmark, coming from the Netherlands," Blinken said.

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"And 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."

Reinforcing air defenses has been crucial for Ukraine as it fights a Russian invasion. Moscow renewed its aerial assaults on Ukraine's national power grid in the spring, causing sweeping blackouts.

President Volodymyr Zelenskyy said last week that he wanted to double Ukraine's air defense capacity over the summer and that his country needed at least seven additional Patriot systems to protect itself.

NATO members have announced the delivery of five additional Patriot and other strategic air defense systems to help Ukraine. Still more aid announcements were expected at this week's summit in Washington, which marks the alliance's 75th anniversary.

A joint statement from the leaders of the United States, Denmark and the Netherlands said the Danish and Dutch governments were in the process of donating American-made F-16s to Ukraine, with the support of the United States.

"We are grateful to Belgium and Norway for committing to provide further aircraft, and to the other members of the Air Force Capability Coalition for their support," the statement said, adding that the group was committed to further enhancing Ukraine’s air capabilities.