(Adds Colombian foreign minister, color)

By Hugh Bronstein

BOGOTA, June 7 (Reuters) - Colombia arrested a Venezuelan national guard officer who authorities said was helping to carry 40,000 AK-47 assault rifle cartridges to Marxist rebels, a charge that could increase already high diplomatic tensions.

The man, identified as Manuel Agudo, was captured on Friday in the southern Colombian province of Vichada, near the Venezuelan border, along with another Venezuelan and two Colombians, Attorney General Mario Iguaran said.

"Four people with 40,000 AK-47 cartridges were arrested, two of them Venezuelans," Iguaran said, adding authorities believed the ammunition was to be sold to the outlawed Revolutionary Armed Forces of Colombia, or FARC.

The arrest comes at a time of discord between the left-wing government of Venezuela and Colombia's conservative president, Alvaro Uribe, Washington's top ally in South America. Venezuela briefly sent tanks to its border with Colombia in March after a Colombian military raid into neighboring Ecuador.

The raid, in which FARC chief Raul Reyes was killed, set off a regional diplomatic crisis. Colombia says files found in Reyes' computer showed links between the FARC and the governments of Ecuador and Venezuela, a charge both countries deny.

Colombia is in a four-decade-old guerrilla war in which thousands are killed, maimed by land mines or displaced every year, according to the government and human rights groups. The conflict, funded for the past 20 years by the multibillion-dollar cocaine trade, often spills into neighboring countries.

Agudo was shown on local television wearing a white T-shirt and a short, military-style haircut as he was brought into the attorney general's compound in Bogota where he will be held.

Colombian Foreign Minister Fernando Araujo told reporters he had asked Venezuela to help with the investigation and expected the neighboring country not only to cooperate but to punish Agudo after "the full weight of Colombian law" has fallen on him.

Venezuela's president, Hugo Chavez, has called Uribe a puppet of U.S. "imperialist" foreign policy, while Colombian officials have long complained Venezuela is not helping to combat the FARC. (Reporting by Hugh Bronstein and Nelson Bocanegra; Editing by Peter Cooney)

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