The soldiers, based in the frontier city of Arauca, were detained after crossing the border as tourists while on leave wearing civilian clothes, the source told AFP, without further comment.

"For now, the case is dealt with on a diplomatic level with an order to Colombia's military not to give any official statements," said a high-ranking government source on condition of anonymity.

Venezuelan National Guard paramilitary police at the border let the visiting Colombians into the country, as they had done in the past, the source said. But they were replaced by Venezuelan army soldiers, who detained the Colombians when they tried to go home.

Colombian military and diplomatic officials began talks Sunday with their Venezuelan counterparts to obtain the soldiers' release.

Relations between the conservative, staunchly pro-US Colombian government of President Alvaro Uribe and the populist, pro-Cuban government of Venezuelan President Hugo Chavez have been tense for years.

Relations between the two countries hit a low point in January following revelations that Bogota secretly hired Venezuelan mercenaries to kidnap a leftist Colombian guerilla leader hiding in Caracas and hand him to Colombian police on the border.

In 2004 Colombia and Venezuela agreed to boost security to stop drug traffickers, leftist rebels and right-wing paramilitary forces active along their 2,219-kilometer (1,379-mile) border.

The governor of the Colombian border state of Arauca, Julio Acosta, urged the Venezuelans Monday to treat the detained soldiers with understanding.

The young men were forced to use Venezuelan highways because of the frequent rebel roadblocks in Colombia, he said.

Just days earlier a soldier was shot dead by leftist rebels when he was traveling in a public transit vehicle, Acosta said.

According to the Venezuelan newspaper Ultimas Noticias, a Colombian soldier was recently arrested in a separate border incident carrying a Venezuelan identification document.

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