Health Minister Violeta Menjivar said at least 1,200 people have been formally diagnosed with the chikungunya viral disease, although the positive testing must still be confirmed by the US Centers for Disease Control and Prevention.
Menjivar, interviewed by state-run Channel 10 television, said that cases were found on the outskirts of the Ayutuxtepeque municipality just outside the capital San Salvador.
In that area at the end of May, the ministry's epidemiologists and infectious disease specialists detected an outbreak of a rare viral disease that caused fever and skin rash, which they said affected at least 181 people.
She said suspected cases were also found in residents in two other area on the edge of northern San Salvador.
The mosquito that transmits chikungunya -- the Aedes aegypti -- is the same one that spreads dengue.
The health ministry has asked people "to eliminate breeding sites" at their homes.
There is no vaccine or treatment for chikungunya, which has infected millions of people in Africa and Asia since the disease was first recorded in 1952.
It has also spread to southern Europe -- with an outbreak in Italy in 2007 and southern France in 2010 -- and arrived in the Caribbean last year, appearing in Martinique and Saint Martin.
Chikungunya produces symptoms similar to dengue, including high fever, joint pain and skin rash.
The disease's name is derived from an east African word meaning "that which bends up," referring to the way that patients are stooped over in pain.