The International Monetary Fund warned on Tuesday that the gap between rich and poor countries was widening amid the pandemic, with low vaccination rates in emerging economies leading to a lopsided global recovery.

The I.M.F. maintained its 2021 global growth forecast of 6 percent in its latest World Economic Outlook report, largely because advanced economies, including the United States, expect slightly faster growth than the global body previously forecast. Economic growth in developing countries is expected to be more sluggish, and the global body said the spread of more contagious variants of the virus posed a threat to the recovery. It called on nations to work together to accelerate the protection of their citizens.

“Multilateral action is needed to ensure rapid, worldwide access to vaccines, diagnostics and therapeutics,” Gita Gopinath, the I.M.F.’s chief economist, wrote in the report. “This would save countless lives, prevent new variants from emerging and add trillions of dollars to global economic growth.”

U.S. economy will expand 7 percent in 2021. The euro area was projected to expand 4.6 percent and Japan 2.8 percent. Rapid expansion was expected for China, at 8.1 percent, and India, 9.5 percent, but both of their outlooks have been downgraded since April. The outlook in China was lowered because of a scaling back of public investment, while India was downgraded because of a severe second wave of the virus slowing the recovery.