The spokesman said North Korea “regards this mugging by the U.S. as an intolerable act of infringement upon the sovereignty” of the country, and demanded an explanation and an apology. The spokesman was not quoted by name, as is common in North Korean news reports.

He did not disclose what the diplomatic package contained.

In a statement, the Department of Homeland Security said its officers assisted in the inspection of three North Korean citizens on Friday, seizing “multiple media items and packages.” It did not explain why the items were seized, but said the North Koreans attempted to retrieve them.

“According to the U.S. State Department, the North Korean citizens were not accredited members of North Korea’s mission to the U.N. and had no entitlement to diplomatic immunity,” the statement said. “The package in question had no protection from inspection.”

North Korea said its delegation had attended a session of the Convention on the Rights of Persons With Disabilities.

The controversy comes at a delicate time in relations between North Korea and the United States. On Tuesday, North Korea released an American college student, Otto F. Warmbier, who is in a coma after 17 months of captivity. American doctors said Mr. Warmbier had suffered extensive brain damage. North Korea said it had freed him on “humanitarian grounds” but did not reveal details of his medical condition.