When President Trump signed an executive order last month temporarily barring visitors from seven mostly Muslim countries, he said he was moving to protect the United States from terrorist attacks. The Homeland Security secretary, John F. Kelly, echoed the president, saying the travel ban was necessary because vetting procedures “in those seven countries are suspect.”

But an internal report written by intelligence analysts at Mr. Kelly’s department appears to undercut the assessment that people from the seven countries — Iran, Iraq, Libya, Somalia, Sudan, Syria and Yemen — pose a heightened threat of terrorism. The three-page report found that “country of citizenship is unlikely to be a reliable indicator of potential terrorist activity.”

The report adds to the difficulties the Trump administration has faced in carrying out the travel ban. Federal judges have suspended the order, and the administration has said it will redo it to withstand legal scrutiny, but has not given a timetable.

The Department of Homeland Security assessment, first reported by The Associated Press, found that only a small number of people from the seven countries had been involved in terrorism-related activities in the United States since the Syrian civil war began in 2011. In addition, the report noted, while terrorist groups in Iraq, Syria and Yemen pose a threat to the United States, militant groups in the other four countries have a more regional focus.