The head of NATO, Jens Stoltenberg, has been one of Ukraine's staunchest supporters since the start of Russia's invasion. He made a surprise visit to Kyiv and says it is in the interest of the alliance to ensure "Ukraine prevails."

Stoltenberg met on Thursday with President Volodymyr Zelenskyy. He promised more military aid. Zelenskyy asked for guarantees of another kind. He said he interprets the timing of the visit as a sign that NATO is ready for "a new chapter."

Zelenskyy said, "We greatly appreciate the military assistance from allied countries. But we want to know when Ukraine will be a NATO member."

Stoltenberg said, "Ukraine's future is in the Euro-Atlantic family. Ukraine's future is in NATO. All allies agree on that."

He invited Zelenskyy to join NATO leaders in July at a summit in Lithuania.

NATO has already provided Ukrainian forces with Leopard tanks. The Netherlands and Denmark have just announced they will send 14 more.

Russian leaders have said one of the goals of what they call their "special military operation" is preventing Ukraine from joining NATO. A Kremlin spokesperson said Ukrainian membership would pose a " significant threat" to Russia's security.