WASHINGTON — The Obama administration on Monday approved a new regulation requiring firearms dealers along the Southwest border to report multiple sales of certain semiautomatic rifles, a rule intended to make it harder for Mexican drug cartels to obtain and smuggle weapons from the United States.

Under the rule, dealers in Arizona, California, New Mexico and Texas will be required to inform the Bureau of Alcohol, Tobacco, Firearms and Explosives if someone buys — within a five-day period — more than one semiautomatic rifle that accepts a detachable magazine and uses ammunition greater than .22 caliber. Such weapons include AK-47s.

Dealers nationwide are already required to report bulk sales of handguns, and the A.T.F. applied to impose such a regulation late last year to help detect bulk “straw buyers” — people who say they are buying weapons for themselves but then transfer them to criminals.

In a statement, the deputy attorney general, James Cole, said the regulation was justified by the need to help the A.T.F. “detect and disrupt the illegal weapons trafficking networks responsible for diverting firearms from lawful commerce to criminals” and in particular to “help confront the problem of illegal gun trafficking into Mexico.”