Turkey has blocked access to more than half a dozen Islamist news websites this week, prompting one group claiming allegiance to Islamic State in Syria and Iraq (ISIS) to accuse Turkey of persecuting Muslims and warn of unspecified retaliation.
Thousands of foreign fighters have travelled through Turkey to join ISIS in the past few years, some of them with assistance from Turkish smuggling networks sympathetic to the militants.
The government in Ankara has been under pressure from its NATO allies to do more to stem the flow and prevent ISIS networks gaining a foothold in Turkey.
A senior government official said the website restrictions, and a series of recent arrests, were an "important part" of efforts to crack down on ISIS recruitment networks.
Several of the websites, which were blocked to access within Turkey from late on Sunday, responded by accusing Turkey of persecuting Muslims and said the telecommunications authority had imposed the ban without giving a reason.
DarulHilafe.com, a Turkish-language site which describes itself as the local desk of ISIS's media wing Al-Hayat, said the decision showed 'the increasing pressure' on Muslims in Turkey and called on authorities to lift the ban.
"They should not forget that if they continue to curb the freedoms of Muslims who have never harmed or attacked Turkey in any way, and increase their pressure, Muslims might retaliate," it said in a statement emailed to Reuters. It did not elaborate.
A female suicide bomber who blew herself up at a police station in Istanbul's historic Sultanahmet district in January, killing one officer, had been in Syria and had links to ISIS, Turkish media reports and diplomats said at the time.
Critics say Turkey has taken too long to block such web sites, contrasting it to the speed with which it imposed temporary bans on YouTube and Twitter at the height of a graft scandal last year, or to the legal action against those accused of insulting President Tayyip Erdogan on social media.
Access to other Islamist websites, including Takva Haber, Tevhidi Gundem, Mustaqim Haber and Enfal Medya was also blocked in Turkey. The regulator could not immediately be reached for comment but the government official said the move was part of Turkey's "commitment to fighting terrorism".
"The term terrorism covers all organizations on the ground that place Turkey's national security and regional stability at risk," the official said.
U.S. Defense Secretary Ash Carter said last week that Washington was still pressing the Turks to "up their game" on closing the border to combat ISIS, a Sunni militant group which has seized large parts of Iraq and Syria.
The Turkish military has tightened security along the border in recent weeks, and the security forces detained dozens of people last week in raids across the country on suspicion of belonging to ISIS.
Late on Monday, Turkish police detained nine more people in the central city of Konya, on suspicion of aiding and recruiting for ISIS, which has declared a caliphate across the territory it has seized.
ISIS has found thousands of supporters in Turkey, some of whom have travelled to Syria to join its ranks. Last month it launched a Turkish-language magazine with a front page story titled 'The Conquest of Constantinople', calling for a new Islamic conquest of Turkey's biggest city Istanbul.
Foreign ministry officials say less than 1,000 Turkish nationals have joined ISIS, but Western diplomats say the figure could be ten times that.
As part of efforts to fight the militant group, the government has banned 15,000 individuals from 98 countries from entering Turkey and deported some 1,500 people suspected of seeking to join the militants.