ACAPULCO, Mexico, July 5 (Reuters) - A convoy was ambushed in remote western Mexico by more than a dozen masked men presumed to be Marxist rebels, killing two policemen and wounding another, officials said on Sunday.

In the latest episode of spiraling guerrilla violence in troubled Guerrero state, the attackers opened fire on five state police as they travelled late on Saturday in an area known as La Montana, about 140 miles (220 km) southwest of Mexico City.

Police said the assailants wore military-style fatigues and fired AK-47 and AR-15 automatic rifles. The men are believed to belong to the Popular Revolutionary Army (EPR) or one of its offshoots, the People's Insurgent Revolutionary Army (ERPI).

"From the way in which they were dressed and carried out the assault, we think the people who carried out this cowardly attack belonged to the EPR," state attorney general Servando Alanis Santos told reporters shortly after the attack.

A state government statement on Sunday said it appeared no rebels were wounded in the half-hour gunbattle and that officials detained three witnesses for questioning.

The ambush was the second following an army attack on a schoolhouse in Guerrero that killed 12 alleged rebels on June 7. Mexican rights groups believe the attack, in which no army troops were wounded or killed, was an outright massacre, with some rebels executed.

Three army troops were later killed and three others wounded in a retaliatory ambush by suspected rebels on June 22 in Guerrero.

"This armed group (EPR) had threatened to take revenge for the deaths of their members killed ... on June 7," the statement said.

At least 50 people have been killed since the EPR first appeared in June 1996 on the anniversary of a peasant massacre by state police, vowing to overthrow the government.

After a rash of attacks in six Mexican states, the group largely called off its ambushes to build grass-roots support, a ceasefire that may have been dumped following the schoolhouse killings.

Mexican President Ernesto Zedillo, struggling to reach peace with the unrelated Zapatista guerrillas in Chiapas state, has branded the EPR as terrorists and refused to negotiate.