Police in a US border city are on heightened alert after receiving a warning from a Mexican drug cartel that officers may be targeted if they carry out off-duty drug busts, authorities said on Tuesday.

Nogales, Arizona police received the threat through an informant after two off-duty policemen seized 400 pounds (182 kg) of marijuana while horseback riding outside the city in early June.

"The warning was ... that the officers, if they are off duty, are to look the other way and ignore any drug trafficking loads that are coming across the border," a police spokesman said.