Prime Minister Jacinda Ardern extended a lockdown in New Zealand's largest city Friday, giving health authorities more time to trace and contain a strain of coronavirus previously unseen in the country.

Ardern said stay-at-home orders would remain in force across Auckland until August 26 in a bid to prevent a mystery outbreak from becoming a fully-blown second wave. 

Genomic tests indicated the latest infections were not the same strain of coronavirus recorded in New Zealand earlier this year.

"This suggests it's not a case of the virus being dormant, or of burning embers in our community, it appears to be new to New Zealand," Ardern said.

She added that tests were also unable to link the outbreak to any cases among quarantined travellers from overseas.

Since four people tested positive on Tuesday -- the first cases of community transmission in 102 days -- New Zealand has now detected a cluster of 30 virus cases.

Amid speculation that the virus could have been brought in via freight, Ardern admitted the source of the strain may never be found. 

But she said it was likely the outbreak had been detected early and expressed hope that it could still be held back.

"We don't necessarily need to answer that question in order to contain and deal with this cluster effectively," she said.

Ardern praised the 1.5 million residents of New Zealand's biggest city as carrying a "heavy load" for the nation's wellbeing.

"Lifting restrictions now and seeing a potential explosion in cases is the worst thing we would do for Auckland and the New Zealand economy," she said.

Health Minister Chris Hipkins said earlier that two cases had been detected outside Auckland despite the lockdown.

But he played down fears that the failure to ring-fence infections meant the virus was destined to spread countrywide.

"The cases so far are connected, they are all part of one Auckland-based cluster, that's good news," he said.

- 'Completely unacceptable' -

The prospect of new wave of infections has rattled New Zealanders, who have come to regard the remote South Pacific nation as a transmission-free haven over the past three months.

Wellington resident Barbara Pond described the situation as "heartbreaking". 

"I'm so sick of hearing about COVID," she said.

"We worked so hard to get rid of it and now every time you turn around it's COVID, COVID, COVID."

National health director-general Ashley Bloomfield acknowledged feelings were running high but urged the public not to take out their frustrations on frontline health workers.

"We've had reports of healthcare workers, who are doing their best to provide testing for people, being verbally abused and even attacked," he said.

"This is completely unacceptable," he added, declining to provide further details.

New Zealand is following the same strategy that helped contain the virus after a seven-week lockdown earlier this year -- isolating positive cases, contact tracing and extensive testing.

Ardern said swabs had been taken from 50,000 people in the past two days.

The government is making testing mandatory for frontline workers at ports and isolation facilities after reports that most staff at Auckland airport had never been checked for the virus.

Hipkins denied the government had failed to secure the border -- regarded as the area most vulnerable to COVID-19's return -- but conceded: "I'd have liked more tests (there) earlier."

Mexico, Peru each top half a million cases

Countries in coronavirus epicenter Latin America reported troubling new statistics on Thursday, with Mexico and Peru both surpassing half a million confirmed infections.

Mexico has 505,293 cases overall after registering 7,371 new infections in 24 hours, the Ministry of Health said. There have been 55,293 deaths, making it the hardest-hit country in the region after Brazil in terms of fatalities.

Infections in Peru have been on the rise since a national lockdown was lifted on July 1, prompting the government on Wednesday to reinstate a Sunday curfew, ban social gatherings and enforce mandatory lockdown in more provinces.

The country has now confirmed 507,996 cases, with a record of 9,441 infections in the last 24 hours, Vice Minister of Health Luis Suarez told a press conference.

There have been 25,648 deaths, he said, a figure that includes 3,658 fatalities from previous weeks now confirmed to be COVID-related.

There were also a record 277 deaths in the last 24 hours.

Colombia, the fourth most affected country in the region, on Thursday reported 433,805 confirmed virus cases.

Deaths have climbed to 14,145, adding 308 fatalities after tabulating data from across the country over the past 10 days, authorities said.

About a third of Colombia's cases are in the capital Bogota, where several areas are under strict confinement orders.

North Korea lifts lockdown of border city

North Korea said Friday it had lifted a three-week lockdown that was imposed on a border city after a defector believed to be carrying the coronavirus sneaked back into the country.

The secretive nation has insisted it has had no cases of COVID-19 -- a claim international experts say is unlikely, given the spread of the disease around the world.

Authorities in July said they were imposing a lockdown on Kaesong city when a 24-year-old man, arrested after crossing the heavily fortified border from the South, was found to be displaying symptoms of the coronavirus.

"It has been proved the situation of anti-epidemic work has been kept and controlled stably", Leader Kim Jong Un was quoted as saying by the state-run KCNA news agency.

Kim "expressed thanks... to the people in the locked-down area for having remained faithful to the measures taken by our Party and government, despite the inconvenience in their living under quarantine".

North Korea's medical infrastructure is seen as woefully inadequate for dealing with any large-scale outbreak.

Pyongyang closed its borders in late January -- the first country in the world to do so -- in a bid to protect itself against the coronavirus.

It imposed tough restrictions that put thousands of people into isolation, but analysts say the country is unlikely to have avoided the contagion.

COVID-19 vaccine will be free for Americans

If a COVID-19 vaccine is proven effective, the US will ensure it's distributed for free to all Americans, officials said Thursday, underscoring there will be no shortcuts on safety.

"We are not at all reducing the regulatory rigor with which we will evaluate and hopefully approve vaccines," Paul Mango, a senior health department official, told reporters. 

Washington has invested more than $10 billion in six vaccine projects and signed contracts guaranteeing the delivery of hundreds of millions of doses should they be approved following clinical trials.

The vaccine doses themselves will be paid for by the government.

Doctors or clinics that administer them will have to be paid but these costs should mostly be covered by private and public insurers.

"Most" commercial insurers have agreed to waive any out-of-pocket costs to their customers, said Mango.

"We are on track to deliver hundreds of millions of doses by January 2021," he added.

Francis Collins, director of the National Institutes of Health (NIH), said he was "cautiously optimistic" that at least one of the six vaccines the US had invested in would come through by the end of the year.

Critics of President Donald Trump have expressed worries that the administration may bypass safety precautions to announce a vaccine is available before the election on November 3 -- a charge Mango denied.

"We are not at all reducing the regulatory rigor with which we will evaluate and hopefully approve vaccines," he said.

Russia this week approved a vaccine even before the start of the last phase of clinical trials, in which the drug is injected into tens of thousands of volunteers to verify its effectiveness and safety.

"I hope that the Russians have actually definitively proven that the vaccine is safe and effective, but I seriously doubt that they've done that," Anthony Fauci, the United States' top infectious disease official said at a virtual panel hosted by National Geographic.