Dr Anne Schuchat of the CDC says swine flu may worsen come autumn
US health officials estimate that at least one million Americans have been infected with swine flu since the H1N1 virus emerged nearly three months ago.
The number is far higher than cases actually reported to the authorities.
The Centers for Disease Control and Prevention (CDC) said many cases were mild, although 127 people had died.
The CDC based its figures on surveys, rather than laboratory evidence, but the numbers suggest the death rate from swine flu is lower than thought.
"We're saying that there have been at least a million cases of the new H1N1 virus so far this year in the United States," said Anne Schuchat of the CDC.
"Reported cases are really just the tip of the iceberg."
The CDC has based its estimate on mathematical modelling, based on surveys by health officials.
If the figures are correct, it is reassuring news, because it indicates that the fatality rate from swine flu is even lower than thought, says BBC medical correspondent Fergus Walsh.
However, Dr Schuchat warned that swine flu might exhibit higher infection rates than seasonal flu and could return in a more virulent form in the autumn.
According to the CDC, there have been 27,717 confirmed or probable cases; some 3,000 people have needed hospital treatment and there have been 127 deaths.
Argentina's hospitals are coming under pressure
Swine flu continues to affect mainly people under 50 years of age, with many of those worst affected having underlying health problems such as asthma or diabetes.
The average age of those who died in the US is 37.
Officials from the CDC and the World Health Organization are watching outbreaks in the southern hemisphere, in particular in Argentina, Chile and Australia, to see how the H1N1 virus has been spreading during the winter months and whether it is likely to become more virulent.
Argentina's health ministry has registered 26 deaths attributed to swine flu, and 1,587 cases. Officials are advising people to try to leave space between each other as they line up to vote in legislative elections on Sunday.
Chilean health authorities say there have been 6,211 cases and 12 deaths.
In Australia, there have been five swine-flu related deaths, all of patients with existing medical conditions, and 3,677 cases, according to official figures.
The H1N1 virus first emerged in April in Mexico, which has recorded 116 deaths and 8,279 cases, according to the WHO.
On 11 June, the WHO declared a global flu pandemic, meaning that swine flu virus was spreading in at least two regions of the world.
Officials stressed that this did not mean the virus was causing more severe illness or more deaths.
According to the latest figures from the WHO, there have been 263 deaths and nearly 60,000 cases in some 100 countries and territories.