Turkish police arrested 29 suspected ISIS terrorists in raids in 20 different locations in the city of Istanbul, announced the state-run Anadolu news agency said on Friday.

Twenty-two of the detainees are foreign nationals, while the rest are Turkish. It did not disclose the details of the other nationalities.

All of the suspects were believed to have fought in ISIS ranks in Syria and were preparing to return to the neighboring country, added Anadolu.

Anti-terrorism police carried out operations in six city districts overnight, it said, adding that one unlicensed weapon and documents belonging to the terror group were found in the raids.

Thousands of foreign fighters have joined the militants in their self-proclaimed “caliphate” in Syria and Iraq in recent years, many of them passing through Turkey.

Turkey has been hit by a series of deadly attacks carried out by ISIS or Kurdish militants and has stepped up anti-terrorism operations across the country. Ankara has detained more than 5,000 ISIS suspects and deported some 3,290 foreign militants from 95 different countries in recent years, according to Turkish officials. It has also refused entry to at least 38,269 individuals.

On Wednesday, Turkish police detained six suspected ISIS terrorists for planning to attack a protest march led by the head of the secularist main opposition party.

In a separate operation on Wednesday, Turkish police detained another 37 ISIS suspects across Turkey, authorities said.

ISIS claimed responsibility for a New Year’s mass shooting at an Istanbul nightclub that killed 39 people.

Earlier, officials said suspected Kurdish militants have detonated an improvised explosive device in southeastern Turkey as the armored vehicle of a district official was passing by. No one was hurt in the attack.

The blast occurred Friday in the town of Baykan in the mostly Kurdish province of Siirt. It came days after two ruling party officials serving in district organizations were killed in attacks also blamed on the outlawed Kurdistan Workers’ Party, or PKK.

Interior Minister Suleyman Soylu said an operation was launched in the area to catch the assailants, adding that such attacks would not “end (Turkey’s) patience and determination” in the fight against the PKK.