Trump made the comments to reporters outside the White House. The Revolutionary Guards said Iranian maritime authorities had requested the capture of the tanker for "not following international maritime regulations."
The incident risks further inflaming tensions between Tehran and the West, which have been increasingly strained since the United States withdraw from a nuclear deal with Iran and imposed sanctions.
Trump said he had also told Republican Senator Rand Paul he could get involved with Iran talks.
Paul had made a proposal to talk to Iranian Foreign Minister Mohammad Javad Zarif, according to U.S. media, but Trump said on Thursday he had not appointed him to any such position.On July 4, British naval forces seized an Iranian tanker in Gibraltar on suspicion of smuggling oil to Syria and breaching European Union sanctions.