North Korean officials said Friday the regime would continue its nuclear buildup as long as South Korea and the United States continued to engage in joint military exercises. Officials said that if the nations continued their military drills, North Korea would not negotiate regarding its nuclear weapons.

“As long as there is continuous hostile policy against my country by the U.S. and as long as there are continued war games at our doorstep, there will not be negotiations,” Han Tae Song, North Korea’s ambassador to the United Nations, told Reuters. “There are continued military exercises using nuclear assets as well as aircraft carriers and strategic bombers and then … raising such kinds of military exercises against my country.”

The U.S. and South Korea began joint military exercises in August, involving thousands of troops and simulations. Pyongyang previously condemned the exercises as “reckless,” claiming they heightened tensions in the Korean Peninsula. Han did not give details about when or if North Korea would test another missile but said it did not plan to give up its program.

“The DPR, my country, will continue to build up its self-defense capability, the pivot of which is nuclear forces and capability for a triumphant…strike as long as U.S. and hostile forces keep up nuclear threat and blackmail,” Han said. “Our country plans ultimate completion of the nuclear force.”

Meanwhile, images taken by satellite earlier this month appeared to indicate that North Korea was rapidly working to build its first operational ballistic missile submarine, according to 38-North, a Washington-based North Korea monitoring organization.

“The presence of what appears to be sections of a submarine’s pressure hull in the yards suggests construction of a new submarine, possibility the SINPO-C ballistic missile submarine – the follow-on to the current SINPO-class experimental ballistic missile submarine,” 38 North reported Thursday.

Tensions have heightened between North Korea and the U.S. in recent months as both countries traded increasingly heated rhetoric. After President Donald Trump’s 13-day trip to Asia, which included a visit to South Korea, North Korean officials slammed Trump for igniting the risk of nuclear war.

“Trump has flown to South Korea as he seeks to strengthen military threats against us and has an intention to light the fuse for a nuclear war,” North Korean newspaper Rodong Sinmun said in a commentary. “The problem is that South Korea is blindly following the U.S. which is intent on the scheme for a nuclear war.”