Two American citizens and a spouse of a Mexican employee were killed Saturday afternoon, a US official said. The official spoke only on condition of anonymity to discuss sensitive information.

The White House said President Barack Obama was "deeply saddened and outraged" by the killings of those linked to the US mission in Ciudad Juarez.

"He extends his condolences to the families and condemns these attacks on consular and diplomatic personnel serving at our foreign missions," the White House said in a statement. "In concert with Mexican authorities, we will work tirelessly to bring their killers to justice."

The State Department authorized US government employees at six US consulates in northern Mexico to send their family members out of the area because of concerns about rising drug-related violence. At least 18,000 people have been killed since Mexican President Felipe Calderon launched an offensive against drug traffickers in December 2006.

The State Department said it would allow family members of diplomatic staff to leave the border cities of Tijuana, Nogales, Ciudad Juarez, Nuevo Laredo, Monterrey and Matamoros.

The department noted the US Embassy in Mexico City has advised American citizens to delay unnecessary travel to parts of the Mexican states of Durango, Coahuila and Chihuahua.

The bloody drug war has plagued the US-Mexican border for years, and once-busy streets are empty after dark. More than 45,000 soldiers have been dispatched to fight cartels since Calderon took office in late 2006, but the US has been critical of Mexican efforts to fight the drug trade amid complaints of human rights abuses.