WASHINGTON — The Trump administration escalated efforts to put pressure on Iran on Monday, taking ambassadors from the United Nations Security Council on a field trip to inspect what American officials called remnants of Iranian missiles and other weaponry illegally supplied to Yemen rebels.

The ambassadors were also White House lunch guests of President Trump, who pressed them to counter “Iran’s destabilization activities in the Middle East.”

The missile fragments, along with other military equipment, were first unveiled last month by Nikki R. Haley, the American ambassador to the United Nations. She presented them as proof that Iran had violated United Nations sanctions on supplies of weaponry to Houthi rebels in Yemen, where more than 10,000 people have died in a war that began three years ago.

At the time, weapons experts said the presentation — of fragments of what military officials said were Iranian-made Qiam missiles, as well as a drone and an antitank missile — had failed to prove conclusively that Iran violated any sanctions.