At least 13 soldiers killed and 56 wounded as bomb detonates when bus carrying off-duty troops passes.

At least 13 soldiers were killed and 56 were wounded when a car bomb hit a bus carrying off-duty military personnel in the central Turkish city of Kayseri.

The attack comes one week after a twin bombing targeted police personnel in Istanbul.

“The bus was full of soldiers going on their weekend leave,” said Al Jazeera’s Sinem Koseoglu, reporting from Istanbul. “The soldiers in the bus are from the commando brigade.”

Turkish authorities have so far detained seven people and are searching for another five in relation to the attack, Interior Minister Suleyman Soylu said.

Saturday’s blast is likely to further anger the Turkish public, who are frustrated by a string of deadly bombings this year, several of which have been claimed by Kurdish fighters, including last week’s Istanbul bombings which killed 44 and wounded more than 150.

INTERACTIVE: Timeline of attacks in Turkey

There was no immediate claim of responsibility, but Deputy Prime Minister Veysi Kaynak likened the attack to last Saturday’s deadly violence outside the stadium of Istanbul soccer team Besiktas, later claimed by an offshoot of the outlawed Kurdistan Workers’ Party (PKK).

“The car bomb attack resembles the Besiktas attack in terms of its style,” he told a group of reporters, adding the incident would not put Turkey off of its goal of fighting the group.

In a statement, President Recep Tayyip Erdogan said a “separatist terrorist organisation” was responsible for the attack, adding that such attacks were not independent of developments in Iraq and Syria.

Erdogan frequently uses the term “separatist terrorist organisation” to refer to the PKK, which is considered a “terrorist group” by the United States, Turkey and the European Union.

Turkey: less than a week after the Istanbul bombing, another car bomb attack kills 13 off duty soldiers in Kayseri. Photo via @cnnturk pic.twitter.com/KOeSiyFQOZ — Piotr Zalewski (@p_zalewski) December 17, 2016

Later on Saturday, a small group of people attacked pro-Kurdish People’s Democracy Party’s (HDP) headquarters in Kayseri.

Protestors managed to get in to the building and set several pieces of furniture on fire, according to Turkish national daily Hurriyet.

Erdogan has previously called for members of the HDP to face prosecution, accusing them of being the PKK’s political wing.

The HDP, parliament’s third-biggest party, denies direct links with the PKK and promotes a negotiated end to the Kurdish conflict, which has claimed hundreds of lives since a peace process, once led by Erdogan and the AKP, collapsed in 2015.

HDP's statement on #Kayseri bombing: "The vortex of violence drags the country into turmoil." pic.twitter.com/stFqjwrEBb — HDP Foreign Affairs (@hdpdiplomacy) December 17, 2016

‘We will fight these cowards’

In Saturday’s blast, the bus was stopped at a red light near the campus of Erciyes University in Kayseri when a car approached it and then detonated, broadcaster NTV said.

Kurdish fighters have previously targeted buses carrying military or security forces.

Defence Minister Fikri Isik said on Twitter that Turkey would redouble its efforts to fight militancy. “We will fight these cowards with a national mobilisation,” he said.

Turkey faces multiple security threats including spillover from the fight against Islamic State of Iraq and the Levant group (ISIL, also known as ISIS) in northern Syria, where it is a member of a US-led coalition against the the armed group.

It also faces regular attacks from Kurdish armed groups, who have waged a three-decade insurgency for autonomy in largely Kurdish southeast Turkey.