US Treasury Secretary Janet Yellen said today that Russia's war in Ukraine posed the "greatest challenge" to the global economy, as G20 ministers prepare to start talks in Indonesia.

"Our greatest challenge today comes from Russia," she told a news conference on the resort island of Bali.

She made her comments ahead of a meeting between finance ministers from the world's top economies and central bank governors on Friday and Saturday.

"The international community must be clear-eyed about the economic and humanitarian consequences of his (President Vladimir Putin's) war," she said.

The conflict caused rapid inflation at a time when the world was struggling to recover from the Covid-19 pandemic, endangering the gains of the past two years and threatening widespread hunger and poverty.

Yellen said she will continue to press G20 allies at the meeting for a price cap on Russian oil to choke off Putin's war chest and pressure Moscow to end its invasion while bringing down energy costs.

But she refused to be drawn on whether Western officials will stage a multi-nation walkout as they did at a G20 meeting in Washington in April.

"It cannot be business as usual," she said. "I can tell you that I can certainly expect to express in the strongest possible terms my views on Russia's invasion... to talk about its impact on Ukraine and the entire global economy and to condemn it."

"I expect that many of my colleagues will do the same," she added.

Russia's finance minister will not attend the Bali talks, instead addressing it virtually, a week after Foreign Minister Sergei Lavrov found himself outnumbered by G20 counterparts in their criticism of Moscow's military assault.