Lawless organised crime gangs are preying on tourists and business travellers in some areas of Mexico, according to a warning from the US State Department.
Criminals are targetting American and European tourists in several states, says the alert.
Crimes include kidnapping, car-jacking and robbery.
Thousands of Americans cross the US-Mexico border every day for business, while millions more from the US and Europe visits the country on holiday.
The State Department explained that although tourist resorts are generally well policed and secure, drug-related violence and crime are rampant in other areas.
In the past two years, 181 US visitors have been murdered in Mexico.
Regular armed skirmishes take place between drugs gangs and police in many towns and cities – often in bars and restaurants during the day.
The gangs have blocked roads with stolen cars, buses and lorries.
“Criminal activity is unpredictable and we recommend that travellers stay away from the worst areas of trouble,” said a State Department spokesman.
“Kidnappings seem to be on the rise. More than 105,000 people were kidnapped in 2012. Last year 130 cases involving Americans were reported to the embassy.”
For a state-by-state breakdown of trouble spots, read the detailed State Department alert
Violence in Africa
The British Foreign and Commonwealth Office (FCO) is warning expats and travellers about the spreading risk of violence in North East Africa.
The problems centre on Yemen and Somalia.
Saudi Arabia and other Middle Eastern countries are carrying out air strikes on Yemen, while Kenya is attacking Somalia from the south.
Yemen and Somalia are both accused of sheltering Islamic terrorist groups and the conflict is spreading in the region following the ousting of the Yemen government.
The situation is made worse as refugees flee to Sudan, which is embroiled in a civil war with South Sudan following the break-up of the nation.
“Violence is sporadic and haphazard,” said an FCO spokesman. “There are places where the British government cannot help travellers in trouble and we recommend no one should go to Yemen or Somalia without an essential reason for the trip.”
Anyone travelling to Turkey, Syria or Iraq with the intention of joining Islamic terror groups risks arrest on their return to the UK, according to a statement from the FCO.
“Terror laws in the UK could mean arrest and prosecution for suspected terrorists,” said a spokesman.