ATLANTA—The number of U.S. swine flu cases has reached nearly 34,000, and deaths have risen 34 percent in the past week to 170, federal health officials reported Thursday.

About four out of five of the swine flu deaths to date were adults aged 25 or older, although seven of the most recent deaths were children, according to the Atlanta-based Centers for Disease Control and Prevention.

The numbers mark an increase from the 127 deaths and nearly 28,000 confirmed and suspected swine flu cases reported last week.

CDC officials believe those cases—which sought treatment and underwent testing—are just the tip of the iceberg. They estimate more than 1 million Americans have been infected with the virus so far, though many probably had only a mild illness. Swine flu is the predominant flu type circulating currently, with ten states reporting widespread cases. The states are California, Connecticut, Delaware, Georgia, Hawaii, Maine, New York, Pennsylvania, Rhode Island and Virginia.

The pandemic was first discovered in California in April, but since then a total of more than 77,000 cases have been reported in more than 100 countries, according to the World Health Organization.