US President Joe Biden has said NATO “would respond” if Russia used chemical weapons in Ukraine.

“We will respond if he uses it. The nature of the response would depend on the nature of the use,” Biden said after a NATO summit in Brussels.

His comments came on an unprecedented day of emergency summits in Brussels, where Western leaders showed a united front against Russia’s invasion.

Mr Biden is travelling to Poland on Friday where more than two million Ukrainians have fled from the fighting.

Biden noted that under his presidency the United States has pledged $2 billion in weapons to Ukraine. He announced a new commitment to “more than $1 billion in humanitarian assistance,” as well as promising to welcome 100,000 of the nearly 3.7 million refugees fleeing the country.

After the United States announced new sanctions, including targeting Russian politicians, Biden said the West was in it for the long haul, intent on “increasing the pain” on Moscow.

Biden intensified the effort to ostracize Putin’s Russia with a call to exclude Moscow from the G20.

“That was raised today,” Biden told reporters, adding that if the group of 20 countries does not agree, then he would press for Ukraine to be allowed to join.

In his address, President Volodymyr Zelensky said that Russia was using phosphorus bombs, which cause severe burns, conducting indiscriminate shelling of civilians, and could resort to “full-scale use” of chemical weapons.

Ukraine has already lived through “a month of heroic resistance. A month of the darkest suffering,” he said. “To save people and our cities, Ukraine needs military assistance without restrictions.”