Updated March 24, 2022 at 2:34 PM ET
President Biden promised allies would respond if Russia uses chemical weapons against Ukraine.
"It would trigger a response in-kind," Biden told reporters. "Whether or not you're asking whether NATO would cross, we'd make that decision at the time."
Biden is in Brussels meeting with NATO allies for emergency talks as Russia's invasion of Ukraine enters its fourth week.
At a press conference at NATO headquarters on Thursday, Biden said that more weapons were flowing into help Ukraine as western leaders met to discuss contingency plans in case Russian aggression intensifies.
Biden said Russian President Vladimir Putin had misjudged the unity of the west.
"NATO has never, never been more united than is it today," Biden said. "Putin is getting the exact opposite of what he intended to have as a consequence of going into Ukraine."
Biden added that he thinks Russia should be ejected from the G-20, but noted that decision will be up to the group. If members object, he thinks Ukraine should be able to attend the next meeting.
Russia had been part of the G-8 group of major economies until it was kicked out for annexing Crimea in 2014.
"Putin is getting exactly the opposite of what he intended to have as a consequence of going into Ukraine," Biden said.
Biden bristled at suggestions that sanctions failed to deter Putin from invading Ukraine
"Sanctions never deter," he said, adding that keeping sanctions in place for the months ahead is "what will stop him."
Biden 's mission ahead of Thursday's meeting was "to ensure we stay united, to cement our collective resolve, to send a powerful message that we are prepared and committed to this for as long as it takes," said his national security adviser, Jake Sullivan.
Earlier in the day, administration officials told reporters the U.S. plans to welcome up to 100,000 Ukrainians and other displaced people fleeing the conflict in Ukraine, and will provide $1 billion in new funding for humanitarian aid for Ukrainians and refugees in neighboring countries.
The Biden administration is also planning new sanctions on Russia, including on members of the Duma and defense companies, is partnering with the G-7 and EU on a new initiative to prevent China or other countries from helping Russian banks do business abroad and will crack down on sales of Russian gold.
From Brussels, Biden heads to Warsaw on Friday to meet with Polish President Andrzej Duda and visit with U.S. troops.
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