Blasts kill 13 outside Istanbul soccer stadium -security sources

* Blast came two hours after end of soccer match * At least one explosion appeared to target police * Dozens wounded, many of them police officers (Adds statement from Erdogan, paragraphs 2-4) By Humeyra Pamuk and Murad Sezer ISTANBUL, Dec 10 (Reuters) - 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. President Tayyip Erdogan described the blasts outside the Vodafone Arena, home to Istanbul's Besiktas soccer team, as a terrorist attack on police and civilians. He said the aim had been to cause the maximum number of casualties. "As a result of these attacks unfortunately we have martyrs and wounded," Erdogan said in a statement. "Nobody should doubt that with God's will, we as a country and a nation will overcome terror, terrorist organisations ... and the forces behind them," he said. Interior Minister Suleyman Soylu said one of the explosions hit directly outside the stadium, while the suspected suicide bomber struck in the adjacent Macka park. 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. CASUALTIES MAINLY POLICE 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. 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. (Additional reporting by Orhan Coskun, Tuvan Gumrukcu, Umit Bektas in Ankara; Writing by Nick Tattersall; Editing by David Dolan and Matthew Lewis)