North Korea conducted a large-scale conventional live-fire military drill in the region of Wonsan on its east coast, South Korea's Office of the Joint Chiefs of Staff confirmed on Tuesday. In a statement, the South's military said it was "closely monitoring" the situation and "maintaining readiness."

Read: North Korea crisis - South Korea remains releaxed

North Korean leader Kim Jong Un was likely to have observed the drill, ostensibly to mark 85 years since the founding of the North's military, South Korean news agency Yonhap reported.

International concerns have been heightened in recent weeks that the North could soon carry out another nuclear test or missile launch, in defiance of United Nations sanctions. The reclusive country, where 40 percent of the population is undernourished according to the Global Hunger Index, often marks significant anniversaries with shows of military force. Although it was feared a nuclear test or ballistic missile launch might happen on Tuesday, no such test happened during the morning.

The harsh rhetoric continued from Pyongyang, with a top military general repeating that the North was ready to use pre-emptive strikes to defend itself.

"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 U.S. imperialists and their vassal forces for aggression," General Pak Yong Sik told a major meeting of senior military and civilian officials.

US nuclear submarine docks in Busan

US President Donald Trump has vowed to prevent North Korea from becoming capable of striking the US with a nuclear missile and ruled out nothing including a military strike. As a show of force, he sent the USS Carl Vinson carrier strike group to waters off Korea. The fleet was carrying out drills with Japanese warships.

South Korea's navy was also carrying out exercises with US destroyers in waters off the Korean peninsula.

As the drills continued, the US Navy said the nuclear-powered guided missile submarine USS Michigan arrived in the South Korean port of Busan on Tuesday. The submarine was not expected to participate in joint naval exercises.

Trump's UN ambassador Nikki Haley said the US wasn't looking for a fight with North Korea's leader Kim Jong Un and wouldn't attack "unless he gives us reason to do something." She praised China's increasing pressure on North Korea, seen by Washington as key to reigning in Pyongyang's military ambitions.

Increased diplomatic activity

Meanwhile, top envoys from Japan, South Korea and the US held talks in Tokyo, with US envoy Joseph Yun saying he and his counterparts agreed to coordinate "all actions" on North Korea. They also agreed China had a key role in pressuring Pyongyang to abandon its nuclear and missile program. According to Japan's foreign ministry, China's envoy for North Korea was also due to visit Tokyo for talks.

Trump on Monday urged UN Security Council ambassadors to consider even tougher sanctions on North Korea. US senators were due to be briefed on North Korea at the White House on Wednesday.