President Joe Biden has told European leaders that the US commitment to the NATO alliance is "unshakeable" and promised to observe the principle that an attack on one member was an attack on all.
His statement was at odds with his predecessor, Donald Trump, who called the 30-member alliance outdated and at one point suggested Washington could withdraw.
"The United States is fully committed to our NATO alliance, and I welcome your growing investment in the military capabilities that enable our shared defences," Biden told an online session of the Munich Security Conference.
"An attack on one is an attack on all. That is our unshakeable vow."
Trump administration officials had publicly hammered, and sought to shame, Germany and other NATO members for not meeting a target of spending 2 percent of their gross domestic output on defence.
Biden's comments signalled a different approach - and one sure to be welcomed by European leaders and NATO officials.
"America's back," Biden told the security conference after his first virtual meeting with Group of Seven world leaders.
"I know the past few years have strained and tested our trans-Atlantic relationship, but the United States is determined - determined - to re-engage with Europe, to consult with you, to earn back our position of trusted leadership," he said.
Biden said the US military was conducting a comprehensive review of its military posture around the world, but he had lifted orders to withdraw US troops from Germany - another decision by the Trump administration that had shocked allies.
In addition, Biden said he had lifted a cap imposed by the previous administration on the number of US forces that could be based in Germany.
German Defence Minister Annegret Kramp-Karrenbauer welcomed Biden's announcement.
"We clearly understand this confirmation of America's commitment to European security, a strong NATO, and a strategically unified West," she told Reuters.
"This signal will be noticed and well understood," Kramp-Karrenbauer said. "It is now up to us to take the hand that Washington has reached out with."
Blinken criticises China over Covid
Meanwhile, US Secretary of State Antony Blinken has told the BBC that the US is fully engaged in helping resolve issues including the pandemic, climate change and Iran's nuclear ambitions.
In his first international interview, Blinken stressed the importance of worldwide vaccination against Covid-19.
He also criticised China for its lack of transparency in uncovering how the coronavirus emerged.
The US is giving $4 billion ($NZ5.4b) to the Covax vaccination scheme, which aims to deliver more than two billion doses to people in 190 countries in less than a year.
"Unless and until everyone in the world is vaccinated, then no-one is really fully safe, because if the virus is out there and continuing to proliferate, it's also going to be mutating," Blinken said.
"And if it's mutating, it's also going to come back and bite people everywhere."
The US has now vaccinated more than 27 million of its own people. However, in many poorer countries, vaccination has yet to begin.
Blinken also accused China of failing to share information that might shed light on the origins of the coronavirus.
A team of investigators from the World Health Organization (WHO) spent four weeks in China on a fact-finding mission at the beginning of 2021. However, two experts from the WHO team said afterwards that China refused to grant full access to the data they sought.
The secretary of state said a better health security system was needed to spot pandemics before they fully emerged.
"It requires countries to be transparent. It requires them to share information. It requires them to give access to international experts at the beginning of an outbreak - things that unfortunately we haven't seen from China," he said.
Different policy on Iran
Under Trump, the US left the deal under which world powers eased crippling economic sanctions on Iran, in return for limits on sensitive activities to show it was not developing nuclear weapons.
Now the US is examining whether to rejoin the deal. President Biden said on Friday that the US must work with other major powers to rein in what he called Iran's "destabilising" nuclear ambitions.
Blinken said the US and its European partners were "once again on the same page" on Iran.
"President Biden has been clear for some time: if Iran returns to its obligations under the nuclear agreement, the United States will do the same thing," he said.
And he said the US would then work with other countries to confront Iran on other issues, including its influence in the region and its ballistic missile programme.
- Reuters / BBC