ISTANBUL (Reuters) - An explosion at a police compound in the south-eastern Turkish city of Diyarbakir that killed three people was the result of a terrorist attack, Interior Minister Suleyman Soylu said on Wednesday (April 12).

Officials had previously said that Tuesday's blast in the biggest city in the mainly Kurdish southeast appeared to have been triggered during the repair of an armoured vehicle.

But Soylu said it had been launched via a tunnel dug beneath the compound. The death toll from the attack has risen to three from an initial one, hospital sources said on Wednesday. "It turned out that the explosion was the result of a terrorist attack. The attack was carried out through a tunnel dug from the outside. They dug a tunnel and placed the explosives in the ground," he said in an interview with broadcaster Haberturk.

He did not say who was believed to be behind the blast, although Kurdish militants have previously carried out similar attacks on police and military compounds in the south-east. Dogan news agency said five people had been detained.

The attack came ahead of Sunday's referendum on changing the constitution and creating an executive presidency long sought by President Tayyip Erdogan. It also marks the first militant attack since the campaigning for the April 16 referendum officially started.

Turkey faces multiple security threats, including from Islamic State and Kurdish militants. The outlawed Kurdistan Workers Party (PKK), which took up arms against the state in 1984, is considered a terrorist organisation by Turkey, the United States and Europe.