Turkish flags, with the control tower in the background, fly at half mast at the country's largest airport, Istanbul Ataturk, following yesterday's blast in Istanbul, Turkey, June 29, 2016. [Photo/Agencies]

ISTANBUL - Turkey announced a day of national mourning on Wednesday for the 42 lives lost the night before during deadly suicide bombing attacks on Istanbul's Ataturk Airport, one of the busiest in Europe.

Across the country flags are flowing at half-mast for the dead, among them 12 foreign nationals from Saudi Arabia, Iraq, Tunisia, Iran, Ukraine, Jordan and Uzbekistan.

A statement released by the Istanbul governor's office also said that of the 239 wounded in the attacks, 109 have been discharged from hospital.

"Everyone is shocked over such a big terror incident," said German journalist Stephen Richter, who arrived in Istanbul on Wednesday to cover the tragedy.

Turkish media reported that Turkey's intelligence units sent a warning letter to related state institutions about potential attacks by the Islamic State (IS) militants nearly 20 days ago.

Hande Firat, CNNTurk's representative in Ankara, said on a live broadcast that Ataturk Airport was listed as a potential target in the letter.

As of now, no group has claimed responsibility for the attacks.

Turkish Prime Minister Binali Yildirim blamed the IS for the atrocity, though he said efforts were continuing to identify the perpetrators.