BOGOTA (Reuters) - Colombia said on Monday it had sent a letter of protest to Caracas after Venezuelan soldiers crossed into its territory last week and detained three Colombian citizens, the latest setback to relations between the two South American neighbors.

Colombia’s foreign ministry said the citizens were arrested on an island in the Orinoco River that has been part of Colombia since 1931. It had conducted an investigation to verify that the incident took place, the ministry said in a statement.

“In the name of the Colombian government the Ministry of Foreign Relations has sent a letter of protest complaining about these kind of actions, which violate national sovereignty, and about the arbitrary detention of three Colombian citizens,” it said.

Venezuela’s socialist government responded later on Monday, saying that a National Guard commission had detected a cocaine lab and arrested three armed Colombians at the site. The statement by the Foreign Ministry called the island by a different name and said it was part of Venezuela.

Colombia was trying to “fabricate” a story as part of its “aggression” against President Nicolas Maduro’s government, Caracas said.

Colombia is the top destination for Venezuelans fleeing food and medicine shortages amidst their country’s deep economic crisis.

Maduro frequently spars with his neighbor’s right-wing government and unauthorized crossings by Venezuelan military personnel along the porous border occur fairly regularly.

Colombia last month said soldiers and helicopters from Venezuela’s Bolivarian National Guard had entered the country without permission, in a “violation to Colombia’s sovereignty.”