An explosion targeting Turkish military buses carrying personnel killed at least 28 people and wounded 61 others in the capital city of Ankara Wednesday, in what government officials called an intricately planned act of terror.

The U.S. Embassy in Ankara activated “duck and cover” following the nearby explosion, a U.S. government source told Fox News.

Officials condemned the attack as “contemptible and dastardly” but did not state how many of the dead or wounded were members of the military. They said┬áthe vehicles were attacked as they stopped at traffic lights.

It was not clear who was behind the bombing. Kurdish rebels, the Islamic State group and a leftist extremist group have carried out attacks in the country recently.

The explosion, believed to be a car bomb, occurred at the height of the evening rush hour in an area some 328 yards away from military headquarters, governor Mehmet Kilicer said. The blast caused a large fire and dark smoke could be seen billowing from a distance.

Turkey’s prime minister Ahmet Davutoglu┬ácanceled a planned visit to Brussels in order to attend a security meeting with President Recep Tayyip Erdogan and other officials.

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