Ecuadorian soldiers clashed with unidentified Colombian militants Thursday on the border, leaving six people killed and another wounded, a senior military official said.

The clash occurred at 5:45 a.m. local time (1045 GMT) in Puerto Mestanza, in Ecuador's northern province of Sucumbios, adjacent to Colombia's Putumayo department.

Leonardo Barreiro, chief of the Ecuadorian armed forces joint command, said the dead were an Ecuadorian lieutenant and five Colombian militants.

Two Colombian militants are under police custody and will be investigated, Barreiro told a press conference. The affiliation of the militants remained unclear.

The leftist guerrilla group, the Columbia Revolutionary Armed Forces (FARC), operates in the area, as do bands of paramilitary groups.