* Turkey unable to keep top secrets despite purge
* Foreign minister calls leak investigation
* Supporters of Erdogan rival deny involvement in leak
* Prime minister faces string of crucial elections
By Ralph Boulton
ISTANBUL, March 28 (Reuters) - Turkey's spymaster discusses possible military intervention in Syria with army and civilian chiefs, and days later it all appears on the Internet.
This breach appears to highlight a shocking truth for Prime Minister Tayyip Erdogan: that Turkey can no longer keep even top-level security planning secret, despite his purge of thousands of officials to root out a covert network of enemies he accuses of sabotaging the state.
Erdogan was out of public action on Friday, resting his voice strained by campaigning for local elections this weekend - the first in a string that will decide the future of a man who has reformed Turkey fundamentally but is now accused of authoritarian and divisive tendencies.
Even without the principal actor, the drama played on over the leaked audio tape that appeared on YouTube on Thursday.
Foreign Minister Ahmet Davutoglu, in whose office the security meeting took place, said "everyone and everything within the Foreign Ministry will be investigated with utmost scrutiny" - a measure of the alarm stirred by the tape and fears of others that might follow.
"This is an act of espionage against the security of the state, it is audacity the like of which we have never seen before," said President Abdullah Gul, an Erdogan ally.
A body close to the Hizmet movement of U.S.-based cleric Fethullah Gulen, whom Erdogan accuses of running a "dirty campaign" of espionage to implicate him in corruption and topple him, said suggestions Gulen was involved were "beyond comedy".
The leaking of such sensitive material, and other evidence of infiltration of the security and surveillance apparatus, could also raise alarm among NATO allies who see Turkey as occupying a crucial position on the edge of a volatile Middle East.
The meeting discussed whether to send forces across the Syrian border to secure the tomb of Suleyman Shah, grandfather of the founder of the Ottoman Empire, in an area largely controlled by militant Islamists fighting the Damascus government. Ankara regards the tomb as Turkish territory.
One leftist daily carried on Friday the headline "I'll Send Four Men, Fire Eight Rockets". This referred to a comment on the tape attributed to intelligence chief Hakan Fidan - one of Erdogan's closest allies - that if necessary he could send four men to Syria with eight rockets and fire onto empty land. This, the voice said, could be used to justify Turkish intervention.
Officials, aware that any involvement in Syria would be highly unpopular with Turkish voters, accept the tape is genuine but say it was manipulated in places. Its dissemination on radio and television has been banned in Turkey and the government ordered that the video-sharing site YouTube be shut down.
A crisis erupted in Turkey on Dec. 17 when anti-graft police raided homes and detained businessmen close to Erdogan and sons of ministers. Erdogan responded by purging members of the police force and judiciary he accused of serving Gulen, a former ally who controls a network of schools and businesses and has over decades infiltrated state institutions and business.
"There are two shocking things in this case," said Sinan Ulgen, Chairman of Istanbul's Edam think tank. "One is the fact that Turkey is now unable to keep a conversation at the highest level in the security establishment...secret."
"The second shocking thing is that despite every measure they have taken since December 17, including the purge of many thousands of people including police, judiciary and probably other places, this continues to go on."
Sunday's local polls will test Erdogan's popularity following the corruption scandal and a heavyhanded police crackdown on anti-government protests in the summer. Failure by Erdogan's AK Party to hold Istanbul, Turkey's biggest city, and the capital Ankara could undermine his authority.
It is unclear yet what effect the corruption scandal and leak will have on the outcome, but government officials argue the tape could work in Erdogan's favour by rallying voters disgusted by the release state secrets behind him.
Sympathetic media stuck up for Erdogan on Friday. "We condemn the heinous treason against Turkey," a couple of dozen pro-government newspapers and television stations said in a joint statement. "Exposing all kinds of state secrets and strategic plans to overthrow the government in pursuit of success, the shadow organisation has given itself away."
Erdogan used Gulen's influence in his early years in power to help rein in an army that had toppled four governments in 40 years. Critics suggest, in essence, that having let the wolf into his kitchen he should not be surprised by the outcome.
The present confrontation began when Erdogan moved last year to close Gulen's schools, a source of income and influence.
ERDOGAN FEELS DUPED
Tercan Basturk, board member of the Journalists and Writers Foundation that often speaks for Hizmet - also known as Cemaat - said the movement had nothing to do with the tape.
"How could Cemaat go into a secure room where four diplomats were in and listen to them?" he asked. "It's beyond comedy. It's a room with all security measures taken, impossible to listen to from outside. They are looking for Cemaat under every stone."
A government official, who asked not to be named, said Erdogan accepted he had been "duped" by Gulen, who denies any involvement in the police graft investigation or leaks. "The main reason for Erdogan's anger now is this sense of having been deceived. He's taking this very personally."
Some feel the portrayal of the power struggle as Gulen versus Erdogan could be too simple.
"One of those attending that meeting is a spy," one nationalist MHP deputy told Hurriyet newspaper. "The government is constantly blaming Cemaat, calling it the parallel structure, but there are various other parallel structures within the state and they are the ones who have recorded this meeting."
The Syria tape as posted alluded to turmoil in state bodies since conflict broke out between Erdogan and Hizmet.
"Currently the state is functioning with a few people and with a few departments able to make proper decisions," Foreign Minister Davutoglu is quoted as saying in discussing the problems of marshalling support for actions.
"Definitely, sir, definitely," Deputy Chief of the Armed Forces General Staff Yasar Guler is quoted as saying.
"Well, are we going to be put off by this?" asks Davutoglu.
"No, we will not be put off, minister, we will not be put off," General Guler is quoted as saying. (Additional reporting by Humeyra Pamuk and Orhan Coskun in Ankara, Seda Sezer, Ayla Jean Yackley and Daren Butler in Istanbul; editing by David Stamp)