NUEVO LAREDO, Mexico, Jan 22 (Reuters) - The Mexican army disarmed Mexican police and mounted street patrols on Tuesday in three cities on the U.S. border on suspicion that agents there work for drug cartels.

Soldiers seized 300 guns from municipal policemen in the city of Nuevo Laredo, across from Laredo, Texas, to check if the weapons were legal or had been used in crimes.

"We expect the inspection to finish in two or three days. In the meantime, the army will patrol," a spokesman for Nuevo Laredo's municipality told Reuters.

Troops conducted similar operations in the cities of Reynosa and Matamoros. All the cities have suffered frequent drug violence in recent years as the local Gulf Cartel fought off a challenge from the rival Sinaloa gang moving into its territory.

Poorly paid police have often been accused of taking sides in the fight between the two cartels, which cost more than 2,500 lives last year.

The army's move came two days after troops arrested a powerful drug lord, Alfredo Beltran Leyva, a key operator for the alliance of smugglers based in the Sinaloa state.

President Felipe Calderon has deployed 25,000 troops and federal police to crush drug cartels in a year-old crackdown that has had mixed results.

The army last year confiscated guns from municipal police in the city of Tijuana.

While scores of traffickers have been arrested, and some extradited to the United States, violence remains on the rise. Drug hitmen last week broke traditional codes of honor against killing children when they murdered a 3-year-old boy and a girl aged 9 in Tijuana. (Reporting by Magdiel Hernandez in Nuevo Laredo, Gabriela Lopez in Monterrey, Writing by Cyntia Barrera Diaz; Editing by David Wiessler)

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