Police arrive at the site of an explosion in central Istanbul, Turkey, December 10, 2016. [Photo/Agencies]

ISTANBUL - Two explosions, one thought to have been a suicide bomb, killed 13 people outside a soccer stadium in Istanbul on Saturday, security sources said, in an attack that appeared to target police hours after a match between two of Turkey's top teams.

Interior Minister Suleyman Soylu described the blasts outside the Vodafone Arena, home to Istanbul's Besiktas soccer team, as a "cruel plot". One of the explosions hit directly outside the stadium, while the suspected suicide bomber struck in the adjacent Macka park, he told reporters.

There was no immediate claim of responsibility. Islamic State, Kurdish and far-leftist militants have all carried out bomb attacks in recent years. The NATO member is part of the U.S.-led coalition fighting Islamic State in Syria, and is battling an insurgency by Kurdish militants in its southeast.

Three separate security sources said at least 13 people had been killed but there was no official confirmation of this. Soylu said he would make a statement on the death toll later.

He had earlier said that around 20 people were wounded and that initial indications were that a car bomb targeting a police bus was responsible for one of the blasts.

"It was like hell. The flames went all the way up to the sky. I was drinking tea at the cafe next to the mosque," said Omer Yilmaz, who works as a cleaner at the nearby Dolmabahce mosque, directly across the road from the stadium.

"People ducked under the tables, women began crying. Football fans drinking tea at the cafe sought shelter, it was horrible," he told Reuters.

A Reuters photographer said many riot police officers were seriously wounded. Armed police sealed off streets. A police water cannon doused the wreckage of a burned-out car and there were two separate fires on the road outside the stadium.

Broadcaster NTV said one of the explosions had targeted a police vehicle that was leaving the stadium after fans had already dispersed. It said 38 people had been wounded.

Turkish soccer team Bursaspor, which finished a match against Besiktas attended by thousands of people two hours before the blasts, said none of its fans appeared to have been injured. It and Besiktas both said they condemned the attacks.

"Those attacking our nation's unity and solidarity will never win," Sports Minister Akif Cagatay Kilic said on Twitter. Transport Minister Ahmet Arslan, also writing on Twitter, described it as a terrorist attack.

President Tayyip Erdogan, who was in Istanbul at the time of the explosions, had been in contact with the chief of Istanbul police and the prime minister and was following developments closely, sources in his office said.

Turkey has been hit by a series of bombings in recent years, some blamed on Islamic State militants, others claimed by Kurdish and far-leftist militant groups.

In June, around 45 people were killed and hundreds wounded when three suspected Islamic State militants carried out a gun and bomb attack on Istanbul's Ataturk airport.