* Departures come after navy chief quits
* Military denies it has been weakened
* PM has moved to distance himself from coup trials
By Jonathon Burch
ANKARA, Feb 7 (Reuters) - Scores of Turkish air force officers have quit since the start of the year, according to opposition lawmakers and media reports, a further sign of weak morale after a top naval commander quit over the jailing of hundreds of his colleagues.
Turkey has detained several hundred serving and retired officers in recent years under Prime Minister Tayyip Erdogan, including as many as a fifth of its top military chiefs, on charges of conspiring to overthrow the government.
The cases are part of an effort by Erdogan, in power for a decade, to stamp out "anti-democratic forces" and bring to heel the once-supreme military, which regularly interfered in politics and staged three outright coups in 1960, 1971 and 1980.
But the detentions have sapped morale in NATO's second-biggest army, which has been fighting a three-decade-old insurgency against Kurdish militants in the southeast and trying to prevent a spillover of the civil war in neighbouring Syria.
The latest reported resignations by 110 air force officers prompted a statement from the office of the chief of general staff, which rarely talks to the media, refuting suggestions that the military had been weakened.
"Because of the dynamic and institutional structure of the Turkish armed forces, any staff member who departs is replaced with another staff member with the same qualification," it said.
January and February were the normal period for military officers to submit voluntary resignations or early retirement requests, the statement said.
Erdogan said late on Wednesday the departures were routine and described suggestions the army had been weakened as "ugly", although he acknowledged last month that lengthy pre-trial detentions were sapping army morale, an apparent bid to distance himself from increasingly unpopular coup trials.
More than 300 past and present officers were handed lengthy prison sentences in September after a 21-month trial on charges of plotting to topple Erdogan's government almost a decade ago.
Hundreds more officers are still on trial in various conspiracy cases and around 37, more than 10 percent, of the 348 generals and admirals in the Turkish armed forces are in jail.
U.S. Ambassador Francis Ricciardone was quoted by a Turkish newspaper this week as criticising the jailing of military leaders. The Foreign Ministry said it had told the U.S. envoy in a meeting on Wednesday that such comments were "unacceptable".
Admiral Nusret Guner, who was operational commander of Turkey's navy, said last month the conspiracy cases and jailings of his colleagues had driven him to quit and that he had feared he would become the next victim. Guner had been due to take over the navy's top role later this year.
Erdogan has received praise at home and abroad for bringing the military under civilian control.
But the years that defendants spend in prison without conviction have raised suspicions the conspiracy trials are aimed at muzzling opposition, with even some sympathisers saying the number of officers charged has spiralled out of control.
About 100 journalists are also in prison, as well as thousands of activists, lawyers, politicians and others. Most are accused of plotting against the government or supporting outlawed Kurdish militants. (Writing by Jonathon Burch; Editing by Nick Tattersall and Andrew Heavens)