The US has voiced its concern at how organised crime is targeting Mexican reporters in an attempt to intimidate them and limit news coverage.
“We are on the side of journalists who are risking their lives,” said Deputy Secretary of State William Burns.
He was speaking a day after the offices of a newspaper in northern Mexico were attacked for the third time in a month.
Pressure groups say Mexico is one of the most dangerous places in the world to be a journalist
“The United States is deeply worried by acts of violence and intimidation that are aimed at limiting the free movement of news,” said Mr Burns during a visit to Mexico City.
Free speech was a cornerstone of democracy, he said, with the work of journalists “vital”.
Self-censorshipOn Sunday, hooded attackers broke into the offices of El Norte newspaper in San Pedro Garza Garcia, near the northern city of Monterrey.
They subdued the security guard, poured petrol in the reception area and set it alight.
No one was hurt but it followed two recent attacks on other regional offices of El Norte.
Dozens of journalists have been killed in Mexico over the past decade, although it is not clear how many were killed as a direct result of their profession.
Pressure groups say that whatever the exact figures, Mexico is highly dangerous for reporters.
Many of the killings have been linked to organised criminal gangs, with journalists targeted because of their coverage of drug-trafficking.
There is also widespread intimidation, leading some media outlets to practise self-censorship to protect their staff.