The blast ripped through a cultural centre in Suruc, a town opposite the Syrian flashpoint of Kobane — which was hit shortly afterwards by a suicide car bombing.
The force of the explosion in Suruc smashed the windows of the building in the centre of the city and set off a fire, witnesses said.
Television footage showed several people lying on the ground covered in blood and ambulances rushing to the scene.
The interior ministry described the blast as a “terrorist attack” and vowed to find the perpetrators as soon as possible and bring them to justice.
“The Turkish authorities have strong reason to believe that the terrorist attack was perpetrated by ISIS,” a government official told AFP, using another name for IS.
The official, speaking on condition of anonymity, said the deadly attack in Kobane “strengthens our suspicions”.
An official in the prime minister’s office said 28 people were killed and nearly 100 injured in Suruc.
“It is a suicide attack,” the official told AFP, speaking on condition of anonymity.
– ‘Targeting Turkey’s unity’ –
Prime Minister Ahmet Davutoglu will send three ministers to the southeastern region following the bombing, his office announced.
“We are calling on everyone to show common sense in the face of this terrorist attack targeting our country’s unity,” the interior ministry said.
In the Kobane attack, a suicide bomber detonated a car bomb at a checkpoint, killing two members of Kurdish security forces, according to Rami Abdel Rahman, director of the Syrian Observatory for Human Rights.
Kobane has been a symbol of resistance against the jihadists since IS fighters were driven out in January.
The attacks came as Turkey was stepping up its role in the fight against the Islamic State group, which has seized large parts of Syria and Iraq over the past year.
In recent weeks, security forces have arrested dozens of IS militants and sympathisers in the most significant action by Ankara against the jihadists.
“It’s now obvious that the Turkish government has upgraded the threat posed by ISIS to among the top ones it is facing,” a senior Western diplomat told AFP last week.
The Suruc blast took place as a group from Turkish left-wing youth associations were preparing to make a press statement in Suruc to announce they would cross into Kobane.
The group was staying at the cultural centre.
– Home to refugees –
Suruc, once a centre of silk-making, is now home to one of the biggest refugee camps in Turkey housing Syrians who have fled the bloody four-year conflict at home.
The camp, which opened in January, shelters about 35,000 refugees who crossed the border after Islamic State jihadists seized Kobane last year.
In January, Kurdish forces backed by rebel groups and US-led air strikes had pushed IS out of Kobane after four months of fierce fighting in a hugely symbolic defeat for the jihadists.
The Islamists make a surprise raid on the town in June but the fighters were driven back by Kurdish forces who took full control of the town.
But IS launched a surprise attack on the Syrian town last month, staging three suicide bombings and re-entering the town.
Many of the injured had been taken to hospitals in Suruc.
Several hundred thousand Syrians have taken refuge in Turkish camps along the border but the vast majority of them are scattered in major cities, where their presence has stoked tensions with locals.
Turkey has long been under international pressure to tighten the security of its volatile 911-kilometre (566-mile) border with Syria to cut the flow of jihadists who try to join the ranks of the Islamic State militants.
Ankara has always vehemently denied claims of Turkish collusion with IS and in turn accused the West of not doing enough to help with the burden of Syrian refugees, 1.8 million of whom are living in Turkey. -AFP