BRUSSELS — The Pentagon on Thursday pressed U.S. allies at NATO to prove they are stepping up their defense of Europe as Washington shifts its attention to security challenges closer to home and in the Indo-Pacific region.

The U.S. has begun cutting the forces it has stationed in Europe — around 80,000 are still deployed — and the Pentagon has launched a review to establish how deep further cuts should be.

A strong U.S. presence is key to NATO’s ability to deter adversaries in Europe, such as Russia.

The cuts will depend on how effectively European allies and Canada respond to a U.S.-demand to overhaul NATO. It would see them take over the conventional defense of Europe. The U.S. would provide backup, including with nuclear weapons, only if needed.

After a briefing at NATO headquarters in Brussels, U.S. Under Secretary of Defense Elbridge Colby praised Germany and Poland, as well as Scandinavian countries, for their efforts.

But he told reporters: “We hope to see more from countries like the United Kingdom.”

Colby said Washington’s aim is to bring “NATO to a more realistic, fit-for-purpose, fit for time, more balanced structure. And so that’s the spirit we’re going to continue pursuing our Europe posture review in.”

The Pentagon has circulated a survey seeking written answers to questions about how allies plan to meet the U.S. aims.

“We just received several of the questionnaires, so we’re looking forward to reviewing them,” said Colby, without saying which countries had responded.

The survey includes demands for information not related to NATO, such as whether allies will grant U.S. forces unfettered access to their bases and airspace. Some in Europe hesitated when the United States launched its war on Iran without consulting them.

“Europe has an interest in making sure that also that Iran doesn’t get a nuclear weapon, which is of course our objective,” Colby said. “We need to be able to have the ability to operate effectively. We need predictability.”