WASHINGTON (NYTIMES) - President Joe Biden on Wednesday (Jan 20) said he expected that President Vladimir Putin of Russia would invade Ukraine, delivering a grim assessment of the ability of the US and its European allies to persuade the Russian leader not to send troops across the border.

"Do I think he'll test the West, test the United States and Nato (North Atlantic Treaty Organisation) as significantly as he can? Yes, I think he will," Mr Biden told reporters during a news conference in the East Room of the White House. 

"But I think he will pay a serious, a dear price for it. And I think he will regret having done it," he added.

The US President's comments came as Russia has marshalled more than 100,000 troops at the border with Ukraine over the past several months.

Mr Biden has vowed extensive sanctions if an invasion happens.

Mr Biden also said it was "not time to give up" on reviving the accord restraining Iran's nuclear programme as talks between world powers drag on in Vienna.

He said some progress was being made, but "it remains to be seen" if Tehran will make a deal.

Mr Biden's negotiators say they have largely agreed on the sanctions that would be lifted in exchange for Iran coming back into compliance with the Joint Comprehensive Plan of Action.

But they are unable to satisfy Iranian President Ebrahim Raisi's demand for guarantees that a future US administration will not quit the deal as former president Donald Trump did in 2018.

Secretary of State Antony Blinken recently told an interviewer that Iran is weeks away from being able to produce enough fissile material for a nuclear weapon, far short of the deal's requirement to limit that breakout time to at least a year.

"It's fair to say that the decision to pull out of the nuclear agreement was one of the worst decisions made in recent US foreign policy history," Mr Blinken told the Pod Save the World podcast.