A mysterious anti-Pyongyang group claiming responsibility for February's raid on North Korea's Embassy in Spain said it is preparing for unspecified "big events," warning the reclusive regime will experience more "humiliation" if its human rights abuses continue.

In a statement posted on its website Sunday evening, Free Joseon urged North Korea to close down its political prisoner camps, halt repatriation of North Korean defectors and begin to reform and open up to the outside world.

"The more the Kim Jong-un regime rejects freedom's orders, such as the dismantlement of political prison camps ... the more humiliation it will experience," the statement said. "We are now preparing for big events. Until then, we will remain silent like the calm before the storm."

It did not elaborate.

The statement came shortly after a North Korean foreign ministry spokesperson denounced the break-in as a "grave terrorist attack" in Pyongyang's first reaction to the incident.

According to a Spanish court, a group of intruders broke into the North Korean Embassy in Madrid, tied embassy employees up, searched the compound and ran away with computers and digital files on Feb. 22.

"We are following the rumors of all hues now in the air that FBI of the United States and the small fry of anti-DPRK 'body' were involved in the terror incident, and so on," the spokesperson said, according to the Korean Central News Agency on Sunday.

Free Joseon, previously known as the Cheollima Civil Defense, has widely been reported to be providing protection for Kim Han-sol, the son of Kim Jong-nam, the assassinated half brother of North Korean leader Kim Jong-un.

In an earlier statement, Free Joseon said it consists of members both in and outside North Korea, and is committed to ending the Kim family's hereditary succession. (Yonhap)