14 suspects were detained on July 12 ahead of the visit to Turkey of U.S. Secretary of State Hillary Clinton, who was due to meet Turkish leaders in Istanbul on Saturday, state media said on Saturday.
State-run Anatolian news agency said the court in the Turkish capital on Friday evening ordered the 14 be remanded in custody, while releasing one further person. One of the suspects had carried out reconnaissance on the U.S. embassy and other diplomatic premises.
Police seized 700 kg (1,500 lb) of ammonium nitrate, a key ingredient in improvised explosives, in a raid on a house in the province of Ankara this week, media reports said.
The police also carried out raids in two other western provinces after the explosives find, the Interior Ministry said.
Media reports have said the suspects, linked to a group active in Afghanistan, were allegedly planning to avenge the killing of al Qaeda leader Osama bin Laden by U.S. forces in Pakistan on May 2.
Turkish police regularly arrest suspected Islamist militants and describe them as having links to al Qaeda, though further details seldom emerge.
Al Qaeda militants were behind bomb attacks in 2003 that killed some 60 people and wounded hundreds in Istanbul.