North Korea will likely conduct its fifth nuclear test around the key meeting of its ruling Workers’ Party later this week, South Korean defense ministry said Monday. Seoul warned last month of its neighbor’s ambitions to carry out the fifth test and an underground nuclear warhead test.

Pyongyang’s seventh congress will begin on May 6, the first in over three decades, triggering speculation over the reason behind the meeting. The reclusive state’s leader Kim Jong Un will head the congress.

“There is a possibility that the North would conduct an additional nuclear test and fire off a (mid- or long-range) missile in a blitzkrieg manner around the party congress,” South Korean defense ministry spokesman Moon Sang-gyun told Yonhap News Agency. He added that Seoul was closely monitoring the situation.

Last month, it was reported that the test for North Korea’s intermediate-range ballistic missile — likely a Musudan — failed. The test came even as North Korea faced strong sanctions from the United Nations Security Council over its fourth nuclear test in January and a rocket launch in February.

The Kim Jong Un-led regime has conducted a series of missile and nuclear tests since the beginning of this year, and the leader has called to strengthen the country’s defense capabilities. Pyongyang has also claimed to have secured intercontinental ballistic missile technologies like reentry and engine technologies, despite stern sanctions.

Besides, North Korea has also threatened to attack the U.S. and South Korea over their joint annual military drills, which it calls a possible cover for a Washington-led invasion. Washington and Seoul have, however, maintained that the drills are for defense purposes.