The plan is part of a broader U.S. campaign to get NATO allies to take on more responsibility for security in the Gulf, including pleas for nations to send ships, aircraft and air defense systems to the region.

The U.S. has already agreed to send three Patriot missile batteries, dozens of fighter jets and other aircraft to Saudi Arabia. And as Esper got a look at one of the Patriot batteries Tuesday at Prince Sultan Air Base, he said Saudi Arabia will "help underwrite" some U.S. costs for the additional aid, which includes about 3,000 American troops.

Standing in front of the battery in the scorching Saudi desert, Esper told reporters traveling with him that he's already had some conversations with counterparts from France, Britain and Germany. "One of my objectives going into Brussels later this week, is to build on those conversations," he said.

The U.S. has dispatched about 14,000 more U.S. troops into the region since May, beefing up efforts to defend the kingdom in the wake of suspected Iranian missile and drone attacks on oil facilities last month. That's despite President Donald Trump's stated goal of pulling troops out of the Middle East and halting American participation in "endless wars."

The Sept. 14 attack on the Saudi oil facilities, as well as earlier ones on a pipeline in the kingdom and ships in the Gulf, stem from Trump's decision to unilaterally withdraw America from Tehran's nuclear deal with world powers and impose crushing sanctions targeting Iran's crude oil sales and shipments.

Allies, however, have been slow to respond to requests to participate in a maritime security effort that is aimed at countering Iranian attacks on tankers and other ships in the region.

Saudi leaders hosted a one-day meeting of defense chiefs from the Gulf region and across Europe this week and used that platform to seek help securing their country.

Gen. Kenneth McKenzie, the top U.S. military commander for the Middle East was at the meeting. Just after Esper landed Monday in Riyadh, McKenzie boarded the plane, and the two men talked for roughly a half hour. Officials did not divulge the content of the meeting, but at least part of it was expected to be a discussion of how to meet Saudi's security requests.

McKenzie likely also gave Esper an update on the U.S. troop withdrawal from Syria and ongoing plans to keep some forces in Iraq, at least temporarily.

Esper's visit to the Saudi air base on Tuesday was his first chance to see the U.S. troops and missile defense systems deployed there. He said the Saudi agreement to help pay some costs of the operation is not unusual. He said he did not know the amount they would contribute.

Asked if taking money for the deployment makes the U.S. a mercenary force, Esper said no because the U.S. is not doing it for the money, but to deter Iran.