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The bus had left Villahermosa in Tabasco state and was en route to Mexico City when it crashed near the municipality of Juan Rodriguez Clara, the Veracruz government said in a statement.
"Apart from being hit, the bus also caught fire, making the job of identifying bodies difficult," Veracruz Governor Javier Duarte said on local television.
The truck was "badly parked on the motorway" when the bus hit, the statement said, citing federal police reports. The bus was completely consumed by the fire, it added, raising the death toll from the accident from 33 reported earlier.
Bus crashes and road accidents occur frequently in Mexico, taking a heavy death toll each year.
In 2012, at least 43 people died when a truck hit a bus in Veracruz, and just a couple of months later more than 30 others were killed in a separate accident in Guerrero state.