"The Boxer took defense action against the drone which had closed into a very close distance, approximately a thousand yards, ignoring multiple calls to stand down and was threatening the safety of the ship and the ship's crew," Trump said at the White House. "The drone was immediately destroyed."
"This is the latest of many provocative and hostile actions by Iran against vessels operating in international waters," he continued. "The United States reserves the right to defend our personnel, facilities and interests, and calls upon all nations to condemn Iran's attempt to disrupt freedom of navigation and global commerce. I also call on other nations to protect their ships as they go through the Straight of Hormuz and to work with us in the future."
In a statement, chief Pentagon spokesperson Jonathan Hoffman said the incident occurred at approximately 10 a.m., local time, when the Boxer was in international waters conducting a planned inbound transit of the Strait of Hormuz, adding that the Iranian "fixed wing unmanned aerial system" approached the Boxer and "closed within a threatening range."
A U.S. official told ABC News that it was the Marines of the 11th Marine Expeditionary Unit that destroyed the Iranian drone using counter-drone jamming equipment.
The Marines from that unit make up several thousand of the approximately 4,500 Marines and U.S. Navy sailors on board the Boxer.
Speaking to reporters at the United Nations in New York City on Thursday, Iran's foreign minister Mohammad Javad Zarif said, "We have no information about losing a drone."
About a fifth of the world's oil passes through the Strait of Hormuz. It was over this strategic waterway where the U.S. said Iran shot down an American drone last month.
The U.S. has also blamed Iran for a June attack on two commercial tankers sailing in international waters in the Gulf of Oman. And Iran's Islamic Revolutionary Guards Corps (IRGC) said on Thursday it had seized a foreign oil tanker with 12 crew members aboard. Iran accused them of smuggling oil in the Persian Gulf.
The Trump administration has been urging allies to join a maritime security initiative to protect the Strait of Hormuz following these events.
Trump's nominee for defense secretary, Army Secretary Mark Esper, said at his confirmation hearing on Tuesday that the initiative, called Operation Sentinel, would involve "passive patrolling in the Strait Hormuz ... to deter provocative actions by the Iranians or IRGC."
"The goal is to increase maritime domain awareness and surveillance capabilities in the region to dissuade malign action," Kathryn Wheelbarger, a senior Pentagon official who briefed NATO allies this week on the proposal, told Reuters.
The State Department and Pentagon are hosting a pre-planned meeting with foreign ambassadors in Washington on Friday to discuss the initiative.
Esper reiterated on Tuesday that the U.S. wants to get back on a diplomatic track with Iran, saying the administration would meet Iran "anytime, anywhere with no preconditions."