President Joe Biden declared the "transatlantic alliance is back" in a powerful speech seeking to reestablish the United States as leader of the West against what he called a global assault on democracy.

The remarks to the annual Munich Security Conference -- held by video link because of the Covid-19 pandemic -- dovetailed with German Chancellor Angela Merkel's welcoming of a return to "multilateralism" after the confrontational years of Biden's predecessor Donald Trump.

Making his first major international address on foreign policy since becoming president in January, Biden said traditional US allies should once again have confidence in Washington's leadership.

"I'm sending a clear message to the world: America is back. The transatlantic alliance is back," he said from the White House.

Biden said his administration was again stressing alliance building, in contrast to Trump's isolationist policies and abrasive treatment of US partners.

"Our partnerships have endured and grown through the years because they are rooted in the richness of our shared democratic values. They're not transactional," Biden said in clear reference to Trump's emphasis on redefining allies as economic rivals.

"We can no longer delay or do the bare minimum to address climate change," Biden said, calling this a "global existential crisis."

But Biden delivered harsh warnings about the threats he said are posed by Russia and China.

The White House said Biden has ditched Trump's controversial efforts last year to reestablish the G8, which became the G7 after Russia was expelled in response to its seizing of the Ukrainian region of Crimea.

Similarly, said Biden, US partners should stand together against "the Chinese government's economic abuses and coercion that undercut the foundations of the international economic system."

On Iran, Biden reiterated his pledge to return to negotiations with Tehran over its nuclear program, but said "we must address Iran's destabilizing activities across the Middle East."

White House has said it has no intention to lift sanctions ahead of talks, the only demand set out by Iranian Supreme Leader Khamenei.