ISTANBUL - Turkish police arrested 12 people in Ankara in an investigation targeting Islamic State jihadists, state-run Anadolu news agency said on Monday, hours after the US embassy there said it would remain closed for the day due to a security threat.The 12 suspects were among 20 people for whom detention warrants were issued by the Ankara state prosecutors' office, Anadolu said. It said they were foreign citizens and had been seeking to recruit new members to the group.Turkish authorities regularly detain Islamic State suspects and it was not clear whether there was any connection between the arrests and the US embassy move. Anadolu said the police operation was "planned previously."On Sunday evening, the US embassy in Ankara said it would be closed to the public on Monday due to a security threat and only emergency services will be provided.It advised US citizens in Turkey to avoid large crowds and the embassy building and to be aware of their own security when visiting popular tourist sites and crowded places. It did not specify what the security threat was that prompted the closure.The Ankara governor's office said additional security measures were taken after intelligence from US sources suggested there might be an attack targeting the US embassy or places US citizens were staying.Turkish police increased operations against Islamic State at the end of 2017 before the first anniversary of a New Year gun attack on an Istanbul nightclub in which 39 people were killed.Islamic State claimed responsibility for that shooting, one of a series of attacks believed to have been carried out by the jihadists in Turkey in recent years.