An off-duty police officer and three suspected gang members were killed in a shootout in El Salvador, authorities said Thursday.

Mara street gang members have been targeting police and killed four during 36 hours.

Vice President Oscar Ortiz, acting as president while Salvador Sanchez Ceren was in Cuba undergoing routine medical testing, said after the shooting that the government will continue pursuing the gangs.

Responding to rumors of another threat against public transportation, Ortiz said, "We are already taking all possible measures and we are going to do everything in our power to prevent small criminal groups from generating economic losses for small, medium and large businesses through terror, intimidation."

Late Wednesday, gunmen attacked elite police force member Daniel Preza Navas as he was moving from his home on the northern edge of San Salvador, the capital. The national police force said Preza Navas apparently was able to return fire, killing three of his attackers before he died in a hail of bullets.

Two police officers were killed when their squad car was ambushed Tuesday. Another police officer was killed Wednesday while on routine patrol in a rural area of the La Paz department, about 50 kilometers (30 miles) southeast of the capital.

Also, on Wednesday, gunmen killed four people on a bus, the latest in a string of bus attacks also attributed to the gangs.

Last week, the gangs paralyzed the public transportation system for almost five days trying to pressure the government to negotiate a new treaty. They also want their leaders moved from maximum security prisons back to common prisons.

But Ortiz said Thursday: "We want to be clear that we are not going to give in to any criminal demand. We are going to continue pursuing these criminal groups."