Passengers on those flights who develop fever or rash on or before Saturday should see a doctor, the Centers for Disease Control and Prevention said.
The warnings came after measles was confirmed in two more adopted Chinese babies recently flown to their new U.S. homes. Four cases were identified last week. Three more are suspected.
Of the six confirmed cases, there were four in Washington state and one each in Maryland and New York.
The newly identified flights were China Southern Flight 327, on March 26, from Guangzhou, China to Los Angeles; Delta Air Lines Flight 484, on March 27, from Los Angeles to Cincinnati; and Delta Flight 518, on March 27, from Cincinnati to Washington, D.C.
Four flights on March 26 had already been flagged: United Airlines Flight 862 and Cathay Pacific Flight CX872, both from Hong Kong to San Francisco; and United Airlines Flights 476 and 784, both from San Francisco to Seattle.
Internationally adopted children are not required to be vaccinated against measles, but health officials say they should be immunized within 30 days of entering the United States. In 2001, a child adopted from China led to 14 measles cases among adopted children and their relatives in eight states.
Measles can cause pneumonia, diarrhea, encephalitis and death. Most people in the United States are immune because of vaccinations.
According to CDC statistics, 91 percent of children between 18 months and three years old were vaccinated in 2001, the last year for which figures are available. There were 116 cases of measles that year, and only one death.
Before the advent of the measles vaccine in 1963, measles infected up to 4 million people a year and killed 450 of them. A more recent outbreak from 1989 to 1991 infected 55,600, leaving 123 dead.