ISTANBUL (AFP) - A Turkish court Friday charged another four soldiers over an alleged 2003 plot to overthrow the government and ordered them to be detained pending trial, Anatolia news agency reported. A general heading Turkey's air force academy was among the suspects charged with "an attempt to remove the government by force" after lengthy questioning, Anatolia said. The arrests bring to 12 the number of soldiers who have been jailed pending trial as part of an expanding probe into the purported coup plot, codenamed "Operation Sledgehammer", according to Anatolia. Among them is four-star general Bilgin Balanli, the head of Turkey's military academies who was in line to become air force commander this year. The investigation, the toughest challenge yet to the once-omnipotent Turkish army, has already landed 196 serving and retired officers in court. Most of them remain in prison. The trial, which opened in December, has been marred by serious doubts over the authenticity of some implicating documents, fuelling mistrust between army and government. Media reports have said the army's annoyance over the expansion of the probe might be behind its surprise decision last month to cancel two major military exercises without explanation, a move that has added to political jitters ahead of Turkey's June 12 general elections. Prosecutors argue the coup plan was drawn up shortly after the Justice and Development Party, moderate offshoot of a banned movement, came to power in November 2002 amid fears it would undermine Turkey's secular system. The soldiers allegedly plotted to bomb mosques, and down a Turkish jet over the Aegean and blame it on Greece, hoping to destabilise the government and garner public support for a coup. The alleged plot leader says papers from a seminar on a contingency plan based on a scenario of tensions with Greece and domestic unrest have been doctored to look like a coup plan.