MEXICO CITY (Reuters) - Mexican police said on Friday they captured a suspected drug kingpin from the Gulf drug cartel during what authorities believe was his own birthday party in a city along the U.S.-Mexican border.

Gilberto Barragan Balderas, 41, was arrested in Reynosa, across from McAllen, Texas, police said in a statement.

The U.S. Drug Enforcement Administration had offered a reward of up to $5 million for information leading to the arrest or conviction of Barragan.

Barragan was in charge of defending some of the Gulf cartel's main smuggling routes into the United States from the rival Zetas cartel, police said.

Founded in the late 1990s by about 40 soldiers who deserted army special forces units to work for the Gulf cartel, the Zetas cartel may now number 10,000 across Mexico and Central America.

The Zetas cartel split from the Gulf cartel in 2010 and it is blamed for some of the worst atrocities in the drug war.

Drug-related violence has killed almost 40,000 people since President Felipo Calderon launched his army-backed assault on cartels after he took office in December 2006.

(Reporting by Cyntia Barrera Diaz; Editing by Will Dunham)