The United States has issued a new travel warning for U.S. citizens in Mexico, urging them to exercise caution amid rising drug-related violence.

It also encourages deferring non-essential travel to Mexican border states, where much of the country's drug-related violence has occurred.

In the updated warning issued Friday, the State Department noted the increased violence in the border region and areas along major drug trafficking routes in Mexico.

The Mexican government said more than 34,000 people have been killed in drug-related violence since December 2006.

The State Department noted that more than 15,000 narcotics-related homicides occurred in Mexico last year, an increase of almost two-thirds compared to 2009. It said the number of U.S. citizens reported to the State Department as murdered in Mexico more than tripled in recent years - from 35 in 2007 to 111 in 2010.

While there is no evidence that U.S. tourists have been targeted, the warning urges citizens to travel only during daylight hours, avoid isolated roads and use toll roads whenever possible.