* Turkish PM doubts Assad will live up to promises
* Deputy PM says Kerry chemical weapons comments a "gaffe"
* Turkey has advocated intervention in Syria (Adds deputy prime minister)
ISTANBUL, Sept 12 (Reuters) - Turkish Prime Minister Tayyip Erdogan said on Thursday he doubted President Bashar al-Assad would fulfil his pledge to put Syria's chemical weapons under international control, accusing him of buying time for new "massacres".
Turkey, one of Assad's fiercest critics, has advocated military intervention in Syria and grown frustrated over what it sees as Western indecisiveness, criticising U.S. assertions that any military strike would be limited in nature.
"The Assad regime has not lived up to any of its pledges, it has won time for new massacres and continues to do so," Erdogan said in a speech in Istanbul. "We are doubtful that the promises regarding chemical weapons will be met."
U.S. Secretary of State John Kerry's seemingly off-hand suggestion on Monday that Assad might avert a U.S. military strike if he surrendered all of his chemical weapons was seized on by Russia, which issued a full-blown diplomatic proposal for putting Syria's chemical stockpile under international control.
Assad's government says it has accepted the plan and Kerry flew to Geneva on Thursday to meet Russian Foreign Minister Sergei Lavrov and hear details on the proposal.
But the government in Turkey, which shares a 900-km (560- mile) border with Syria and is sheltering a quarter of the 2 million people who have fled the country, is sceptical.
"Unfortunately U.S. Secretary of State Kerry's remarks on surrendering chemical weapons removed the possibility of an intervention," Deputy Prime Minister Bulent Arinc told Turkey's NTV television in an interview.
"We are not banging the drums of war. But something had to be done against a tyrannical administration that is responsible for the deaths of more than 100,000 people, has used ballistic missiles and now chemical weapons," he said.
"Kerry's remarks served Russia and Assad. The United States couldn't back out of it, and to me, it was a big gaffe."
Erdogan said late last month any military strike should be aimed at bringing an end to Assad's rule and not a "24 hour hit-and-run". He has cited the NATO operation against Serbia during the Kosovo war as a possible template.
Turkey has said previously it would be ready to take part in any international action against Assad, even outside the auspices of the United Nations, and has put its armed forces on alert to guard against threats from Syria.
Arinc ruled out Turkey acting alone. "It's out of the question that we would directly intervene," he said.
(Reporting by Ayla Jean Yackley and Daren Butler; Writing by Jonathon Burch; Editing by Nick Tattersall/Mark Heinrich)