Pyongyang marked the 85th anniversary of the founding of its
military on Tuesday by reportedly conducting a large artillery drill.
South Korea's Yonhap news agency, citing a South Korean government source, said there were signs that the North's military carried out the live-fire exercise in areas around the city of Wonsan on its east coast.
South Korea's defence ministry could not immediately confirm the report.
At a "national meeting" of thousands of senior military and civilian officials the day before, Pak Yong-sik, North Korea's defence minister, reiterated that the country is ready to use pre-emptive strikes or any other measures it deems necessary to defend itself against the "US imperialists".
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North Korean leader Kim Jong-un did not attend. It was not known how he was marking Tuesday's anniversary.
North Korea often marks significant dates by displaying its military capability and has conducted five nuclear tests on such occasions in the past. Pyongyang launched a missile one day after the 105th birthday of late founder Kim il-Sung on April 15.
Al Jazeera's Divya Gopalan, reporting from Seoul, said that Tuesday's reportedly large artillery test could be a way of North Korea "saving face and managing tensions" by taking action that is likely outside the UN sanctions remit and is at a less provocative level than a missile launch or nuclear test.
"So this [artillery test] is not banned by the UN, yet it is some kind of provocation," Gopalan said.
North Korea's recent moves are testing the developing policies of US President Donald Trump, who has reportedly settled on a strategy that emphasises increased pressure on North Korea with the help of China, the North's only major ally, instead of military options or trying to overthrow North Korea's government.
The nuclear-powered USS Michigan submarine arrived at Busan in what was described as a routine port visit to rest the crew and load supplies. Commander Jang Wook from the South Korean navy public affairs office said there is no plan for any drill.
The submarine's arrival comes as the USS Carl Vinson aircraft carrier heads toward the Korean Peninsula in a show of force.
Al Jazeera's Gopalan said that the mood in Seoul was anxious and that, while there have often been tensions between South Korea and its neighbour over the past decades, there is a general sense that the level has been ratcheted up recently.
"Due to the fact that the US is putting weight behind its words and maybe because the US seems to be almost as unpredictable North Korea at the moment; so people are finding it hard to gauge where this is going to go," Gopalan said.
She reported that presidential elections are due in South Korea and there are many important domestic issues that would normally be the focus of public debate.
"Instead, North Korea is the main topic of conversation," said Gopalan.
In Tokyo, US representative for North Korea Policy Joseph Yun met on Tuesday with his Japanese counterpart Kenji Kanasugi and Kim Hong-kyun of South Korea.
Al Jazeera's Scott Heidler, reporting from Tokyo, said no policy change announcements are expected from the meeting, which is an opportunity for the three countries to plan and coordinate their response to any potential action by North Korea.
"It's not unprecedented that they sit down like this, but obviously with this current situation, it's much more important," he said.
A US ministerial meeting has been scheduled in Washington DC for Friday. 
Meanwhile in an unusual event, the entire US Senate has been asked to attend a briefing on North Korea on Wednesday at the White House.
"So obviously this has escalated to a higher level because the situation is is so tense," said Heidler.
Japan's foreign ministry also announced that China's envoy for North Korea, Wu Dawei, will visit Tokyo on Tuesday for talks with Kanasugi, which may take place later this week.
Trump spoke by phone with both the Japanese and Chinese leaders on Monday. Chinese state broadcaster CCTV quoted President Xi Jinping as telling Trump that China strongly opposes North Korea's nuclear weapons programme and hopes "all parties will exercise restraint and avoid aggravating the situation".
Trump met the 15 UN Security Council ambassadors, including from China and Russia, at the White House on Monday, and said the UN Security Council must be prepared to impose new sanctionson North Korea.
"The council must be prepared to impose additional and stronger sanctions on North Korean nuclear and ballistic missile programmes," he said.
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 intercontinental ballistic missile (ICBM) launch is in its "final stage"