As the hostile atmosphere in the region grows ever more intense, Tuesday marked the 85th anniversary of the founding of the North Korean army. To celebrate the anniversary, dictator Kim Jong Un oversaw what is thought to be the country’s largest ever live-fire military drill. 300 miles south, a U.S. nuclear submarine carrying Tomahawk missiles docked in the South Korean port of Busan — a move which prompted Pyongyang to threaten a “merciless nuclear strike” against its enemies.

The growing tensions in the region have led the White House to take the unusual move of briefing all 100 senators this week, while Secretary of State Rex Tillerson will chair a special meeting of the UN Security Council which will allow members to “show their resolve to respond to further provocations with appropriate new measures.”

Despite satellite imagery appearing to show preparations, North Korea did not conduct a sixth nuclear test, surprising those who had predicted one would take place to mark the anniversary.

“Signs are detected that North Korea’s military is conducting a large-scale drill around the eastern port city of Wonsan on the anniversary,” a South Korean government source told the Yonhap news agency. The source claimed the exercises included 300-400 long-range artillery pieces, which would have put Seoul and nearby areas within range. The South Korean military said it was monitoring the situation and “firmly maintaining readiness.”

At the same time as Kim was overseeing the live fire drill, the USS Michigan submarine docked in the port of Busan, in what officials called a routine visit. The sub is set to join an incoming group of warships led by aircraft carrier Carl Vinson, currently steaming towards the region. The USS Michigan is a nuclear-powered sub carrying 154 Tomahawk cruise missiles and 60 special operations troops and mini-subs, according to South Korean newspaper Chosun Ilbo.

It and the Carl Vinson will take part in ongoing joint military exercises being conducted by the U.S. and South Korea. Known as Foal Eagle, the exercises are seen by North Korea as practice for an invasion.

North Korea called the latest military manoeuvres by the U.S. an “undisguised military blackmail,” which would plunge the region into a “touch-and-go situation.”

An editorial in the state-run Rodong Sinmun newspaper said: “If the enemies recklessly provoke the DPRK, its revolutionary armed forces will promptly give deadly blows to them and counter any total war with all-out war and nuclear war with a merciless nuclear strike of Korean style.”

In a surprise move, the White House announced it would be briefing all 100 senators on Wednesday on the situation in North Korea. The move came after the U.S. President met with the ambassadors from UN member states on the security council to discuss the threat from Pyongyang’s ongoing development of nuclear weapons and missile tests.

“The status quo in North Korea is unacceptable and the council must be prepared to impose additional and stronger sanctions on North Korean nuclear and ballistic missile programs,” Trump said. “North Korea is a big world problem, and it’s a problem we have to finally solve.”

Trump’s Secretary of State Rex Tillerson will chair a special meeting of the UN Security Council on Friday to discuss ways “to maximize the impact of existing Security Council measures,” a spokesman said.

South Korea’s special representative for Korean peninsula peace and security affairs Kim Hong-kyun, met Tuesday with U.S. and Japanese counterparts to discuss the best way to bring China and Russia into the fold and gain their support for countering North Korea’s actions. Following the meeting, U.S. envoy for North Korea policy Joseph Yun said: “We believe China has a very, very important role to play.”

Japanese Prime Minister Shinzo Abe could push for Russian help with the situation when he meeting President Vladimir Putin in Moscow on April 27, though the Kremlin has refused to say what the meeting will focus on.

“We hope that all parties, including Japan, can work with China to promote an early peaceful resolution of the issue, and play the role, put forth the effort, and assume the responsibility that they should,” Chinese foreign ministry spokesman Geng Shuang told reporters in Beijing.