* N.Korea missile tests described as 'serious, unlawful'
* US, S.Korea, Japan to up trilateral defence cooperation
* Experts and officials say North ready for nuclear test
* Pyongyang urged to accept aid as Covid-19 spreads
North Korea's recent missile tests were "serious, unlawful" provocations, senior officials from South Korea, the United States and Japan said on Wednesday, urging Pyongyang to return to dialogue and accept offers of Covid-19 aid.
South Korea Vice Foreign Minister Cho Hyun-dong, US Deputy Secretary of State Wendy Sherman and Japanese Vice Foreign Minister Takeo Mori made the comments as they gathered in Seoul, three days after North Korea conducted the latest in a series of missile tests.
The three-way meeting of the countries' No. 2 diplomats, the first such gathering since November and the first since South Korea's President Yoon Suk-yeol took office in May, highlighted the urgency and gravity of North Korea's intensifying weapons tests.
Mori's visit also marked such trip by the Japanese vice foreign minister since late 2017 amid strained bilateral ties over issues including Japan's occupation of the Korean peninsula and war-time labour.
Seoul and Washington officials have said North Korea is ready for what would be its first nuclear test since 2017, which Sherman has said would trigger a strong and clear response.
The trio urged Pyongyang to abide by international sanctions and immediately cease actions that "escalate tensions or destabilise the region," a joint statement said.
They also pledged to ramp up trilateral security cooperation to curb the North's threats, with Sherman reaffirming the US defence commitments, including extended deterrence.
"They stressed that a path to serious and sustained dialogue remains open and urged the DPRK to return to negotiations, while also expressing their hope that the DPRK will respond positively to international offers of assistance to fight against Covid-19," the statement said, referring to North Korea by its the initials of its official name, the Democratic People's Republic of Korea.
North Korea has carried out at least 18 rounds of weapons tests this year, underscoring its evolving nuclear and missile arsenals.
In its latest test, North Korea fired eight short-range ballistic missiles, likely its largest single launch, a day after South Korea and the United States ended joint military drills involving an American aircraft carrier.
The allies launched eight surface-to-surface missiles on Monday in their own show of force responding to the North's test.
South Korea's President Yoon Suk-yeol, who took office in May, and US President Joe Biden vowed at their recent summit to deploy more US strategic military assets as part of efforts to bolster the extended deterrence.
North Korea has been grappling with its first confirmed coronavirus outbreak since last month. It has reported more than 4.2 million patients with fever symptoms among its 25 million population, but never confirmed how many tested positive for the virus, lacking in test kits and medical supplies.
Seoul and Washington said they had respectively offered Covid aid but Pyongyang did not respond, even as the World Health Organization warned of a worsening Covid-19 situation there.
"The United States remains prepared to meet the DPRK without preconditions and we iterate again, we have no possible intent toward the DPRK," Sherman told a joint news conference.
Seoul officials have said Pyongyang has conducted multiple experiments with a detonation device in preparation for its seventh underground nuclear explosion.
The nuclear test could come as early as next week ahead of a planned plenary meeting of the ruling Workers' Party's powerful central committee, some analysts said.
"The meeting is primarily designed to review economy and other policy issues, but it could also touch on nuclear policy," said Cheong Seong-chang, director of South Korea's Sejong Institute's North Korea studies centre, referring to previous meetings held shortly after nuclear tests in 2013 and 2017.
US Deputy Secretary of State Wendy Sherman, South Korea's First Vice Foreign Minister Cho Hyun-dong and Japan's Vice Minister for Foreign Affairs Takeo Mori attend a joint news conference at the Foreign Ministry in Seoul. Jean Heon-Kyun/Pool via REUTERS