BRUSSELS—President Biden met with NATO leaders Thursday to agree on new measures to help Ukraine battle Russia’s invasion and address growing concerns Moscow might use chemical, biological or other unconventional weapons in its monthlong war.

The North Atlantic Treaty Organization’s emergency summit is the first in a day-long string of gatherings Mr. Biden has planned with European allies and other world leaders, as they map out their next steps helping Ukraine defend against the attack launched last month by Russian President Vladimir Putin.

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