Ankara: Four people were killed and 14 others wounded in an attack at the Turkish Aerospace Industries' headquarters on Wednesday, the government said, and witnesses said they heard gunfire and a loud explosion at the site near Ankara.

Interior Minister Ali Yerlikaya said two attackers were killed in what he called a terrorist attack, adding three of the injured are in critical condition. TV broadcasters earlier showed footage of armed assailants entering the TUSAS building.

"Two terrorists were neutralised in the terror attack on the TUSAS Ankara Kahramankazan site. Sadly, we have 3 martyrs and 14 wounded in the attack," Yerlikaya said, referring to the Aerospace Industries. Turkish President Tayyip Erdogan, alongside Russia's Vladimir Putin at a BRICS conference in the Russian city of Kazan, also called it a terrorist attack.

The cause and perpetrators of the blast and subsequent gunfire remained unclear. No group had claimed responsibility. Prosecutors have launched an investigation, and the state-run Anadolu Agency reported. Some media reports claimed a suicide attack had occurred and that there were hostages inside the building, though officials have not confirmed this.

Witnesses told Reuters that employees inside the building were taken by authorities to shelters and no one was permitted to leave for a few hours. They said the blasts they heard may have taken place at different exits as employees were leaving work for the day.

Broadcasters showed images of a damaged gate and footage of an exchange of gunfire in a parking lot, as well as attackers carrying assault rifles and backpacks as they entered the building. Ambulances and helicopters later arrived.

TUSAS is Turkey's largest aerospace manufacturer, currently producing a training craft, combat and civilian helicopters, as well as developing the country's first indigenous fighter jet, KAAN. Owned by the Turkish Armed Forces Foundation and the government, it employs more than 10,000 people. NATO Secretary General Mark Rutte condemned the attack and said the military alliance would stand with its ally Turkey. The European Union delegation in Turkey also condemned the attack.