The sons of imprisoned drug kingpin Joaquin "El Chapo" Guzman were probably behind a grenade attack on a military convoy in northern Mexico on Friday, killing five soldiers, authorities said.

The gunmen, who also fired several rounds from assault rifles, freed a wounded suspect who was being escorted by the soldiers in an ambulance during the pre-dawn ambush in Culiacan, capital of the state of Sinaloa.

Guzman's sons were "very probably" responsible for the attack, which freed a suspect identified as Julio Oscar Ortiz Vega, alias "El Kevin," General Alfonso Duarte, a regional commander, told reporters.

The gunmen stole the ambulance while a military Humvee caught fire in the assault, which left 10 people injured, including a Red Cross worker. Authorities initially said four soldiers died by the general said five were killed.

The soldiers were returning with the wounded suspect after another shootout in the mountain community of Bacacoragua, part of the municipality of Badiraguato, the region where Guzman and other notorious drug lords were born.

The suspect was "being transferred to Culiacan for immediate medical attention ... but unfortunately this group acts in a premeditated, cowardly, treacherous way, using firearms and grenades," Duarte said.

The general said Guzman's brother, Aureliano Guzman Loera, alias "El Guano," is battling for control of drug production against the Beltran Leyva drug cartel in the remote region. Two of Guzman's sons, Jesus Alfredo and Ivan, are wanted by the authorities.

The gunmen had waited in several vehicles in the northern part of Culiacan before launching their attack, said Sinaloa state government secretary general Gerardo Vargas Landeros.

"The soldiers were taking a wounded person who had participated in a clash with the army and they rescued him, taking him and the ambulance," he said.

Sinaloa is one of Mexico's most violent states and the ambush came as murders are on the rise this year across the country.

- Guzman woes -

Guzman is hailed as a Robin Hood-type figure in his stronghold, and his cartel is still considered a fearsome force despite his capture.

The cartel leader was arrested in the Sinaloa coastal city of Los Mochis in January, six months after he escaped from prison near Mexico City by sneaking through a 1.5-kilometer (one-mile) tunnel that opened into his cell's shower.

Guzman now waits for a Mexican judge to decide whether he can be extradited to the United States, a ruling that he could appeal, although US officials believe he could be in US territory before the end of the year.

His capture dealt a blow to his cartel, but the gang still has a strong leader, his longtime associate, Ismael "El Mayo" Zambada, who has never spent a day in prison.

Guzman's sons are also accused by the authorities of having roles in the criminal group.

But in a sign of the cartel's vulnerability, one of them, Jesus Alfredo Guzman Salazar, 29, was kidnapped in a restaurant in the Pacific resort of Puerto Vallarta in August, apparently by members of the rival Jalisco New Generation drug cartel.

Two US government officials, who requested anonymity because they were not authorized to speak publicly, have told AFP that the son was released days later and the reasons for the abduction are unclear.

One of the US officials said the kidnapping appeared to be a "non-sanctioned event" committed by lower ranking members of the New Generation cartel, and that their bosses ordered them to release Guzman's son.