Patriot air defense systems, missiles and ammunition are among the American-made weapons NATO allies will buy under an arms deal brokered with President Trump to help Ukraine defend itself from Russian attacks, officials say.

Nearly all of the weapons are immediately available to ship to Ukraine, officials said, meaning they are either from existing military stockpiles or have just been built.

Mr. Trump portrayed the new agreement as lucrative for the United States, despite giving few details on how it would be enacted. “It’s a very big deal we’ve made,” Mr. Trump said on Monday from the Oval Office alongside Mark Rutte, NATO’s secretary general.

“You have very wealthy countries buying the best equipment in the world, and we have the best equipment in the world,” Mr. Trump said. “We make equipment like no other.”

Mr. Rutte said that at least eight NATO countries were ready to pay for the arms and praised Mr. Trump for helping Ukraine obtain “what it needs to have to maintain, to be able to defend itself, against Russia.”

“But you do want Europeans to pay for it, which is totally logical,” Mr. Rutte said.

More Ukrainians were killed in June than in any other single month so far in the three-year war, the United Nations reported. Russian forces continue to advance in eastern Ukraine.