Beware of leg clots, acting US surgeon general has warned. Dangerous blood clots may affect up to 600,000 Americans a year.

Dr. Steven Galson launched a campaign Monday to get patients and doctors to recognize the potential emergency.

Deep-vein thrombosis or DVT is caused by sitting rigidly for too long, which can cause a blood clot to form in a deep vein, usually in the calf, thigh or groin. If the clot breaks loose and journeys to the lungs, it may lead to pulmonary embolism — blocking an artery in the lungs and reducing oxygen supply.

"It's a silent killer. It's hard to diagnose," Dr.Galson said. "I don't think most people understand that this is a serious medical problem or what can be done to prevent it."

The World Health Organization has said that passengers travelling for four hours or more face double the risk of developing the clots. The agency recommends that travellers get up for a short walk and exercise their calves by moving their feet and ankles hourly.

In Monday's report, the surgeon general and the Agency for Healthcare Research and Quality said travellers aren't the only ones at risk. The report's authors estimated between 350,000 and 600,000 Americans have the clots each year, and at least 100,000 deaths may be directly or indirectly related to the clots.

People are at high risk for the clots if they:

Have had surgery recently.

Are over age 65.