A dozen American nonprofit groups work regularly in North Korea. None have a permanent presence there, and must travel back and forth to do their work. Many have said they are now being denied permission to travel, according to Keith Luse, executive director of the National Committee on North Korea, a pro-engagement group in Washington.

Talk of a travel ban on aid groups has been discussed among North Korea watchers since last week.

“It has become clear that the Trump administration regards the provision of humanitarian assistance to the North Korean people as a legitimate target for its maximum-pressure campaign,” Mr. Luse wrote in an email to about 200 people last Thursday. “Indeed, a line has been crossed.”

Mr. Luse urged the group’s members and supporters to persuade the administration to lift the new restrictions. The problems were reported last week in The Wall Street Journal.

Aid groups also described confusion over a Treasury Department regulation that was issued last March. It required nongovernmental organizations to be licensed if they had “partnerships and partnership agreements” with either the North Korean government or others who are under the international sanctions.

“In terms of current policies, with the kinds of constraints we’re facing, we’re not able to continue in the same way,” said Linda Lewis, director of the North Korea program at the American Friends Service Committee, a Quaker group that works on agriculture programs with North Korean farms.

“There are things we historically did that are now difficult,” she said. “The travel ban makes it hard to plan and responsibly monitor our projects.”

Ms. Lewis had planned to travel with two Chinese co-workers to North Korea in November, but her application for a State Department travel exception was rejected within days; she was informed there was no way to appeal the decision.