SEOUL, South Korea — North Korea said it has no plans to invite anyone from the U.S. to discuss the case of Pae Jun Ho, the American citizen sentenced to 15 years of hard labor for unidentified “hostile acts” against the communist country.

Pae was arrested for “many different acts of crime” including plotting to overthrow the North Korean regime, an unidentified Foreign Ministry spokesman was cited as saying by the official Korean Central News Agency.

“We have no plans to invite anyone from the U.S. over the problem of Pae,” the spokesman said, vowing that North Korea will “sternly respond to illegal acts of U.S. citizens.”

The verdict against Pae, announced May 2, increased the tension over North Korea’s nuclear program that has escalated since President Kim Jong Un in February conducted an atomic test. Patrick Ventrell, a U.S. State Department spokesman, last week called for the immediate release of the American citizen.

In the remarks carried by state media, the unidentified North Korean Foreign Ministry spokesman said Pae entered North Korea "with a disguised identity in an intentional way under the back-stage manipulation of the forces hostile toward" the country.

The exact nature of Pae's alleged crimes has not been revealed. Friends say Pae is a devout Christian and tour operator based in China who traveled frequently to North Korea to feed orphans.

Pae, a Korean-American known in the U.S. as Kenneth Bae, was arrested after he entered the border city of Rason as a tourist on Nov. 3 and was tried on April 30, KCNA said earlier.

Yang Moo Jin, a professor at the University of North Korean Studies in Seoul, said last week the Pae verdict, “the strongest sentence the communist country has ordered for a U.S. detainee,” was issued to get the attention of the international community.

Separately, North Korea said South Korea should stop all provocative and military acts if it wants to normalize the joint Kaesong industrial complex, according to a statement carried by KCNA which cited an unnamed spokesperson at the National Defense Commission.

The last remaining seven South Korean workers at the industrial zone jointly run with the North left the complex on May 3, closing one of the last channels of inter-Korean contact.

North Korea recalled all of its workers from the factory park on April 8 to protest U.S. and South Korean joint annual military drills which concluded last week. The decision by South Korean President Park Geun Hye to withdraw workers was “inevitable” after North Korea refused to hold talks over the facility, Unification Minister Ryoo Kihl Jae said on April 26.