More than 3.3 million Americans had signed up for new insurance plans by the end of last month under President Barack Obama's troubled signature health care law, officials said Wednesday.

The administration also reported a slight increase in the proportion of younger people signing up for coverage—an important factor as young healthy people are needed to bankroll coverage for older, sicker patients.

"It is very, very encouraging news, we are seeing a healthy growth in enrollment," said Health and Human Services Secretary Kathleen Sebelius.

The launch of the new law, a centerpiece of Obama's presidential legacy, was hampered by failures of the federal website set up to offer new plans to Americans.