Turkey has blocked access to more than half a dozen Islamist news websites this week, prompting one group claiming allegiance to Islamic State to accuse Turkey of persecuting Muslims and warn of unspecified retaliation. Thousands of foreign fighters have traveled through Turkey to join Islamic State in Syria and Iraq in the past few years, some of them with assistance from Turkish smuggling networks sympathetic to the militants, Reuters reported. The government in Ankara has been under pressure from its NATO allies to do more to stem the flow and prevent Islamic State 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 Islamic State 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 Islamic State'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 Islamic State, 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.