Angry Colombia says Venezuela sent troops into its territory

President Juan Manuel Santos (C) chairing a meeting with Foreign Minister Maria Angela Holguin (1-L), Defense Minister Luis Carlos Villegas (1-R) to review information about alleged Venezuelan military incursion near the border between both countries

Colombia slammed its neighbor Venezuela on Thursday for what it said was an "unacceptable" incursion by Venezuelan military forces on its territory, and sent troops to secure the area.

The move came just months after the two countries started to reopen their border following a security dispute.

Colombia's President Juan Manuel Santos said he had complained to his Venezuelan counterpart Nicolas Maduro that Venezuelan troops had been camped out in the Colombian border region of Arauca since Tuesday.

Shortly afterward, Santos said the Venezuelan troops had withdrawn -- but not before triggering a tense row between the two neighbors.

"I told (Maduro) that the situation is totally unacceptable for Colombia," Santos said of his conversation with his Venezuelan counterpart, after meeting with his defense chiefs.

"He assured me that he had ordered the withdrawal of the troops from Colombian territory."

Santos said Colombian "military personnel" had been sent to the area.

"I have instructed the armed forces to stay in the area and continue exercising full sovereignty over the territory."

The foreign ministry said in a statement earlier that it had "information from the Colombian armed forces about the installation of a Venezuelan military camp in the municipality of Arauquita," in Arauca.

It said the government had sent officials to the area to investigate and meet Venezuelan border authorities.

The two countries in August started reopening border crossings after the frontier was closed a year earlier.

Venezuela had closed the border following an armed attack on a Venezuelan military patrol that left three soldiers wounded.

That attack was blamed on Colombian paramilitaries, remnants of Colombia's long civil war.

Colombian authorities also reported a further incident in January 2016 in which gunshots were exchanged in the area.

Arauquita is a strategic spot near the two countries' porous border, located on a road linking the Caribbean with the Pacific.