A suspected Islamic State suicide bomber killed at least 28 people, mostly young students, in an attack on a Turkish town near the Syrian border on Monday.
Television footage showed bodies lying beneath trees outside a cultural center in the mostly Kurdish town of Suruc in southeastern Turkey, some 10 km (6 miles) from the Syrian town of Kobani where Kurdish fighters have been battling Islamic State.
The blast tore through a group of mostly university-aged students from an activist group as they gathered to make a statement to the local press about a trip they were planning to help rebuild Kobani. The Hurriyet daily said the attacker was an 18-year old woman, but there was no confirmation.
"Our initial evidence shows that this was a suicide attack by Islamic State," one senior official in Ankara told Reuters.
A second official also said Islamic State appeared to have been responsible and that the attack was a "retaliation for the Turkish government's efforts to fight terrorism".
Turkey's NATO allies have been seeking tighter controls on a porous border with Syria that serves as a frontline in the battle against Islamic State. But monitoring is difficult with 1.8 million Syrian refugees now settled on the Turkish side.
"I saw more than 20 bodies," one witness told Reuters by telephone, giving his name as Mehmet. "It was a huge explosion, we all shook."
Video footage showed young men and women standing behind a banner declaring support for Kobani, some holding up small red flags. Suddenly there was a huge explosion, apparently from within the crowd, sending up a column of flame.
The attack comes weeks after Turkey deployed additional troops and equipment along parts of its border with Syria, concerned about the risk of spillover as fighting between Kurdish forces, rebel groups, Syrian government troops and Islamic State militants intensifies.
Turkey's leaders have said they do not plan any unilateral military incursion into Syria but have also said they will do whatever is necessary to defend the country's borders.
President Tayyip Erdogan said 28 people were killed in Suruc. The death toll could rise. "Terror has no religion, no country, no race," he said of the bloodiest such attack in Turkey since at least 50 people were killed in the town of Reyhanli near the border in 2013.
"UK stands shoulder to shoulder with Turkey in condemning unequivocally all terrorism," British ambassador Richard Moore said on his Twitter account. "I have kids that age."
Ankara fears any disorder in the border area could re-ignite an armed Kurdish separatist rebellion by the Kurdistan Workers Party (PKK) that has killed some 40,000 since 1984.
Turkey's Kurds have been enraged by what they see as Ankara's failure to do more to stop Islamic State. The PKK held the government responsible for Monday's attack, saying it had "supported and cultivated" Islamic State against the Kurds.
Kobani was the site of one of the biggest battles against Islamic State last year and was secured by Syrian Kurdish fighters last month after repeated assaults.
Syrian Kurdish forces, known as the YPG, drove the Islamic militants back from the town with the help of U.S. air strikes, after months of fighting and siege.
The students from the Federation of Socialist Youth Associations had been planning a trip to Kobani to build a library, plant a forest and build a playground, Fatma Edemen, a member of the group wounded in the blast, told Reuters.
"I was behind a banner so I couldn't see the attacker, but we understand it was a suicide attack. I was thrown to the ground... I jumped up and began running before I even realized I was hurt," Edemen, a 22-year-old journalism student at Ankara University, said by telephone.
The blast came after a series of attacks on the Kurdish HDP party in the run-up to a June 7 election, including two small bombings at a political rally in the city of Diyarbakir, which the party blamed on Islamic State sympathizers.
"What gives us pause about this attack is that while the others were haphazard and sloppy ... the size of this explosion suggests something more sophisticated," said Aaron Stein, an Atlantic Council fellow who specializes on Turkey and Syria.
"That would suggest organization beyond a lone wolf cooking up something in their kitchen."
(By Seyhmus Cakan; Additional reporting by Ayla Jean Yackley in Istanbul, Orhan Coskun, Ece Toksabay and Dasha Afanasieva in Ankara, Tom Perry in Beirut; Writing by Nick Tattersall and Daren Butler; Editing by Ralph Boulton)