Answering growing frustration over vaccine shortages, President Joe Biden announced that the U.S. is ramping up deliveries to hard-pressed states over the next three weeks and expects to provide enough doses to vaccinate 300 million Americans by the end of the summer or early fall.

Biden, calling the push a “wartime effort,” said Tuesday the administration was working to buy an additional 100 million doses of each of the two approved coronavirus vaccines. He acknowledged that states in recent weeks have been left guessing how much vaccine they will have from one week to the next.

Shortages have been so severe that some vaccination sites around the U.S. had to cancel tens of thousands of appointments with people seeking their first shot.

“This is unacceptable,” Biden said. “Lives are at stake.”

He promised a roughly 16% boost in deliveries to states over the next three weeks.

The administration said it plans to buy another 100 million doses each from drugmakers Pfizer and Moderna to ensure it has enough vaccine for the long term. Even more vaccine could be available if federal scientists approve a single-dose shot from Johnson & Johnson, which is expected to seek emergency authorization in the coming weeks.

The Centers for Disease Control and Prevention reported that the government plans to make about 10.1 million first and second doses available next week, up from this week’s allotment of 8.6 million. The figures represent doses of both the Pfizer and Moderna vaccines. It was not immediately clear how long the surge of doses could be sustained.