North Korea is excavating a new tunnel at its nuclear test site in the country’s northeast, a Seoul official said Friday, fanning concerns over another possible provocation ahead of a series of summits and high-level diplomatic events here.

Increased movement of people and vehicles has recently been spotted at the Punggye site, along with hints of construction work for a new tunnel, the official said, requesting anonymity due to the sensitivity of the matter.

No signs of an immediate blast were detected, however, he added.

Pyongyang carried out three rounds of underground experiments there ― in 2006, 2009 and 2013.

The South Korean government declined to confirm the news, citing intelligence.

“We are closely monitoring North Korea’s nuclear activities in cooperation with the U.S. and other related countries,” Unification Ministry spokesperson Jeong Joon-hee said at a press briefing.

The new construction activities came as Seoul gears up for its first trilateral summit with China and Japan under President Park Geun-hye on Sunday. South Korea’s Defense Minister Han Min-koo is also scheduled to meet with his U.S. counterpart Ash Carter on Monday for annual security consultations.

The four countries are members of the stalled six-party talks aimed at denuclearizing North Korea.

Though the possibility of a fourth blast appears low for now, Pyongyang has in recent months threatened to test its long-range missile or “nuclear deterrent” in protest against what it calls the hostile policy of Washington and its allies.

By Shin Hyon-hee (heeshin@heraldcorp.com)