|More than 34,000 people have been killed so far in Mexico's drug-cartel violence [Reuters]
Authorities in Mexico have pulled 37 bodies from a mass grave in the north of the country and are continuing to dig in the area.
Federal police digging in the outskirts of Durango, capital of the Mexican state of the same name, uncovered the severely decomposed bodies.
Police found 26 bodies on Wednesday and 11 on Thursday.
The local prosecutor's office said in a statement late on Thursday that the 11 bodies included one dead woman.
The tally brings to more than 57 the number of bodies pulled from at least two ditches in the area in the last two weeks, according to local media.
Mexico has been engaged in a bloody war against drug traffickers for more than four years. Many of the 37,000 people killed have been found buried in ditches.
The US state department broadened its travel warning on Mexico on Friday to include parts of five additional states, including a highway where suspected drug gangs shot two US customs officials in February.
|Click here for more of Al Jazeera's special coverage
The warning advises US government personnel and American citizens to defer nonessential travel in certain parts of Jalisco, Nayarit, San Luis Potosi, Sonora and Zacatecas.
It outright bans US employees from traveling to Colotlan and Yahualica, two cities in the central-west state of Jalisco near the Zacatecas border due to increasing drug gang violence.
"Concerns include roadblocks placed by individuals posing as police or military personnel and recent gun battles between rival transnational criminal organizations involving automatic weapons," the state department warning said.
The restrictions were added to a previous warning against travel throughout the states of Tamaulipas and Michoacan and to parts of the states of Chihuahua, Coahuila, Durango and Sinaloa.
Escalating drug violence
This month authorities found mass graves containing 145 people in the border state of Tamaulipas.
Durango is part of the so-called Golden Triangle of northern states - along with Sinaloa and Chihuahua - that has seen a sharp rise in unrest since authorities began a crackdown on drug traffickers who use the region to grow marijuana and opium poppies, and transport their wares.
The Tamaulipas killings - apparently of passengers on public buses in the region - are being blamed on the drug gang, the Zetas.
Durango has been a battleground in a bloody turf war between the Sinaloa cartel and the Zetas gang.
Earlier this month, authorities arrested 16 police officers suspected of protecting four members of the Zetas cartel, which has been blamed for the massacres.
In another development, Mexico's army announced earlier this month that it had captured a man who has allegedly confessed to involvement in the killing of seven people, including a well-known poet's son.
The killing of Juan Francisco Sicilia on March 28 sparked demonstrations across the country against the violent drug war in the country, which has claimed more than 34,000 lives so far.
Circumstances surrounding the arrest, however, cast doubt over whether Rodrigo Elizalde Mora, the suspect, was coerced into the confession.
The army says that Mora was captured in the central city of Cuernavaca, where the killings occurred, on Thursday.
He confessed to working for the South Pacific Cartel, led by Hector Beltran Leyva, and to help kill Sicilia and a group of friends, an army statement said.
Mora, however, appeared before reporters badly beaten, and said he had been kidnapped by four unidentified men in a 4x4 vehicle, who had then turned him over to the authorities.
"They picked me up and they beat me," he told local media.
The chain of events is similar to a case in 2008, when three suspects were quickly arrested for a grenade attack on a crowd celebrating Mexico's independence.
Eight people died in that attack, and the suspects who were arrested said they were kidnapped by a drug cartel, beaten and left for the authorities.