ISTANBUL - A fighter proclaiming allegiance to Islamic State has appeared in a video urging fellow Turks to rebel against "infidel" President Tayyip Erdogan and help conquer Istanbul, highlighting the threat the NATO member faces as it battles the radical insurgents.Turkey has been in a heightened state of alert since launching a "synchronized war on terror" last month, which included air strikes against Islamic State fighters in Syria and the opening of its air bases to US-led coalition forces.The moves marked a major policy shift by Ankara after years of reluctance in taking a frontline role against the Islamist fighters pressing on its borders. The steps followed a suicide bombing, blamed on Islamic State, in the Turkish border town of Suruc on July 20 which killed 34 people.Speaking in accent-free Turkish, the Islamic State fighter, a rifle propped against his body, accused Erdogan in the video of "selling the country to crusaders" and of allowing US access to Turkish bases "just to keep his post.""Turkish people: without losing any time you have to rebel against these atheists, crusaders and infidels who have made you slaves," he said, as two other armed and bearded men, their heads covered with turbans, sat silently beside him.It also described Kurdish PKK militants in Turkey, some of whom have joined the fight against Islamic State in Syria, as "atheists" and warned that eastern Turkey would fall into Kurdish hands unless Islamist fighters rose up.Reuters could not verify the authenticity of the video, which made no specific threats of attacks. But the SITE Intelligence Group, which tracks extremist activity, described the recording as a call by a Turkish Islamic State fighter for support from Muslims in Turkey.The video, which was widely cited on social media and reported on by mainstream Turkish media outlets, was entitled "A Message to Turkey" and said to have been produced by the "Raqqa state media office", a reference to the Sunni hardline group's de facto capital in northern Syria."Altogether and under the orders of Abu Bakr al Baghdadi ... let's conquer Istanbul, which the traitor Erdogan works day and night to hand over to crusaders," the man said, referring to Islamic State's leader.The video, which emerged late on Monday, comes two months after Islamic State launched a Turkish-language magazine with a story entitled "The Conquest of Constantinople" on its cover, urging a new Islamic conquest of Turkey's biggest city Istanbul.TURKEY EXPOSEDThe United States carried out its first manned air strikes against Islamic State from Turkey's southern Incirlik Air Base last week and officials have said operations will be ramped up once additional coalition forces arrive.Ankara and Washington are working on plans to provide air cover for Syrian rebels and jointly sweep Islamic State fighters from a strip of land along the Turkish border in a bid to cut off one of the group's lifelines.Turkey has already carried out three of its own air strikes in northern Syria after one of its soldiers was killed in a cross-border firefight with Islamic State, while authorities have tried to break up the group's networks in Turkey with a series of raids and mass arrests in recent weeks.Islamic State long used the porous Turkish border to bring in foreign fighters and supplies and appeared not to target Turkey in return. But security experts say the jihadists will be re-evaluating that understanding as air strikes from Turkish bases begin to take their toll."Turkey is a lot more exposed now, particularly as they can't easily back away from the commitment on (the United States) using their bases," one Western diplomat said.A senior Turkish government official said security forces seized around 30 suicide vests in the first half of this year that intelligence suggested were going to be used by Islamic State militants in attacks on targets including police stations.Following an online crackdown by Turkish authorities in which more than half a dozen Islamist news web sites were blocked, at least one group that claimed allegiance to Islamic State had already warned of retaliation.Islamic State has seized large parts of Syria and Iraq, both of which border Turkey, declaring a caliphate within the territory it rules. It has attracted thousands of foreign fighters from around the world.The group's strict interpretation of Islam has also found thousands of supporters in Turkey. Foreign Minister Mevlut Cavusoglu has said around 700 Turks have joined the group but Western diplomats say the figure could be far higher.The 7-minute video, shot in what appeared to be a semi-desert location, included archive footage of Erdogan at a press conference with US President Barack Obama and greeting Saudi King Salman. Speaking over the footage, the fighter accused Erdogan of being an ally to "Americans, Jews, Crusaders, and infidel agents of the al Saud family."