North Korea has held major live-fire drills as it marks the anniversary of the founding of its military.
The exercise took place in an area around the North's eastern coastal town of Wonsan, according to the South Korean military, as a US guided-missile submarine arrived in South Korea amid rising tensions over Kim Jong-un's regime.
Envoys from the United States, Japan and South Korea have also met in Tokyo to discuss the growing threat posed by North Korea's nuclear weapons and missiles programme.
Experts thought North Korea might conduct a nuclear test or a ballistic missile launch to mark the anniversary, but as of Tuesday evening neither has occurred.
Crowds in Pyongyang, the North Korean capital, laid flowers and paid respects at giant statues of the country's former leaders Kim Il-sung and Kim Jong-il, one day after the minister of defence reiterated that the North was ready to use pre-emptive strikes or any measures it deems necessary to defend itself against "US imperialists".
Gen Pak Yong Sik told a meeting of thousands of senior military and civilian officials: "The situation prevailing on the Korean Peninsula is so tense that a nuclear war may break out due to the frantic war drills of the US imperialists and their vassal forces for aggression."
South Korea's Joint Chiefs of Staff said they are closely watching North Korean military action in the Wonsan city area, where the drills are reportedly being held.
South Korea's Yonhap news agency said the exercise involved 300 to 400 artillery pieces, but an official from Seoul's defence ministry could not confirm these details.
The streets of Pyongyang were quiet. Floral tributes and bowing at statues and portraits of the leaders is a regular routine on major anniversaries.
People also gathered in open spaces to take part in organised dancing, another common way to mark holidays.
Choe Un Byol, who came with his family to the bronze statues of the former leaders, said: "Our great leaders founded and wisely led our revolutionary army, and just like that, now our respected Marshal Kim Jong Un is leading wisely, so even though the situation is tense, we are celebrating the day."
North Korea often also marks significant dates by displaying its military capability. It launched a missile one day after the 105th birthday of late founder Kim Il-sung on April 15.
Recent US commercial satellite images indicate increased activity around North Korea's nuclear test site, and third-generation dictator Kim Jong Un has said the country's preparation for an ICBM launch is in its "final stage".
The USS Michigan, a nuclear-powered submarine, arrived at the South Korean port of Busan in what was described as a routine visit to rest the crew and load supplies. Cmdr Jang Wook from South Korean navy public affairs said there is no plan for a drill.
The submarine's arrival comes as the USS Carl Vinson aircraft carrier headed toward the Korean Peninsula for a joint exercise with South Korea. The US 7th Fleet said two American destroyers are conducting simultaneous maritime exercises with naval ships from South Korea and Japan.
Despite the build-up, US president Donald Trump has reportedly settled on a strategy that emphasizes increased pressure on North Korea with the help of China, the North's only major ally, instead of military options or trying to overthrow the government in Pyongyang.
Mr Trump told ambassadors from UN Security Council member countries that they must be prepared to impose additional and stronger sanctions on North Korea.
He said: "This is a real threat to the world, whether we want to talk about it or not.
"North Korea is a big world problem, and it's a problem we have to finally solve. People have put blindfolds on for decades, and now it's time to solve the problem."
