Officials were seen removing remains from the charred wreckage of the bus, which had been traveling from the city of Guadalajara in the western state of Jalisco to Los Mochis in Sinaloa.
"Twenty one lifeless bodies have been counted," Sara Quinonez, attorney general of Sinaloa state, said in a video posted on social media, adding that it would take time to identify the victims.
The truck and the bus -- carrying nearly 50 people -- collided head-on before catching fire, Roy Navarrete, director of civil protection in Sinaloa, said at a press conference.
A double-decker passenger bus collided with a trailer on the Mazatlan-Culiacan Highway in northern Mexico, Sinaloa state Attorney General Sara Bruna Quinonez Estrada confirmed.
In a message posted on social media platform X, the head of the Sinaloa State Attorney General's Office said on Tuesday: "So far, according to the expert work that is still being carried out in that area, 21 lifeless bodies have been registered" at the site of the accident in the municipality of Elota.
"We still do not have the total number of people who were travelling; we are in the process of comparing it with the (bus) company's list," she added as quoted by Xinhua news agency report.
The bus burst into flames following the impact and bodies were burnt beyond recognition, so it will take time "to verify the identification" of the fatal victims, she said.
In a later statement, Sinaloa's government said the bus was travelling between Guadalajara and Los Mochis when the accident occurred at 5:15 a.m. local time on Tuesday.
The trailer overturned on the road when the bus drove into it, leading both vehicles to catch fire.The cause of the accident has not been verified, according to local media
Authorities were investigating the cause of the pre-dawn accident on a highway in the municipality of Elota.
Deadly road smashes are common in Mexico, often due to high speeds, poor vehicle conditions or driver fatigue.Crashes involving freight trucks have also increased on the country's highways.
In one of the worst accidents in recent years, at least 29 people were killed in July 2023 when a passenger bus careened off a mountain road and plummeted into a ravine in the southern state of Oaxaca.
In November, 12 people died and dozens injured when a bus overturned in the southeastern state of Veracruz.
Safety campaigners have called for stricter regulations, such as a ban on trucks pulling two cargo trailers.
