By Jonathan Landay

WASHINGTON (Reuters) -The number two U.S. diplomat questioned the need for NATO in a post on X - which he later deleted - as the alliance prepares for an annual summit expected to be dominated by a U.S. demand for higher defense spending and Russia's war on Ukraine.

Deputy Secretary of State Christopher Landau was replying to a social media thread by Matthew Whitaker, the U.S. ambassador to NATO. Whitaker in his post said that what happened in the Indo-Pacific mattered for transatlantic security.

"He obviously didn't get the memo our [sic] of our Deputies Committee meeting on this very issue," Landau wrote at 6:56 pm on Wednesday, referring to Whitaker. "NATO is still a solution in search of a problem."

It was not clear if Landau meant for his message to be public or if he intended to send Whitaker's post to a third person.

"This was a casual, lighthearted remark intended for a brief, private exchange," a State Department spokesperson said in emailed comments.

"The Deputy Secretary’s comment was in the context of his desire to improve NATO and ensure it remains focused on its mission," the spokesperson said.

Landau's post was later deleted. The Deputies Committee is subordinate to the National Security Council and composed of deputy secretaries of State, Defense and other agencies, according to the White House website.

The administration of former President Joe Biden had pushed the European allies to pay more attention to the threats in the Indo-Pacific, particularly from China, but the Trump administration has encouraged those allies to focus on their own security.

Landau's post contrasts with views of most of NATO's European members, who see Russia and its war on Ukraine as an existential threat, and the U.S.-led alliance as their main means of defense.

His post came as NATO leaders prepare for a two-day summit in the Hague beginning on June 24, where they will consider U.S. President Donald Trump's demand that they boost defense spending to 5% of GDP.

Ukrainian President Volodymyr Zelenskiy is expected to attend the meeting. Since Russia's full-scale invasion in February 2022, he regularly has attended NATO summits, and alliance members have pledged billions in weapons and condemned Russia for an illegal war of conquest.

Trump has shifted U.S. policy, partially accepting Moscow's justifications for its full-scale invasion in February 2022 and disparaging Zelenskiy.

European allies have expressed concern about Trump's commitment to the 32-member alliance and its support for Ukraine's battle against Russia's occupation forces.

(Reporting by Jonathan LandayAdditional reporting by Daphne Psaledakis; Editing by Humeyra Pamuk and Leslie Adler)