* Group of Western, Arab states, Turkey to meet on Saturday
* Opposition source: Arms for insurgents is main agenda item
* West: Meeting more about rebel unity, rejecting extremism
* Turkey calls for "humanitarian corridor" in Syria conflict
By Mariam Karouny
BEIRUT, April 19 (Reuters) - Syria's opposition says it hopes their international backers meeting in Istanbul on Saturday will give teeth to a tacit agreement that arming rebel groups is the best way to end the dynastic rule of President Bashar al-Assad.
The 11-nation "core group" of the Friends of Syria, including the United States, European and Arab nations, has been deadlocked over how to remove Assad, whose security forces killed and arrested thousands of protesters who took to the streets to demand democratic reforms in March 2011.
More than 70,000 have been killed in the revolt and subsequent civil war. But a military stalemate has set in and much of Syria left in ruins because of a divided and ineffective opposition, a lack of action by foreign allies and Assad's ability to rely on support from Russia, Iran and China.
A senior Syrian opposition source who took part in preliminary meetings in Istanbul before the conference said that Saturday would be a "turning point".
"The main reason behind this meeting is to arm the Syrian rebels. The (Friends of Syria) have acknowledged our right to defend ourselves, now they have to provide us with the means," the source said on condition of anonymity as he did not want to say on-record that an agreement has almost been reached.
At the last conference in Rome, the United States said it would give non-lethal aid directly to rebels for the first time and more than double its support to Syria's civilian opposition.
A communique from that meeting said participants "underlined the need to change the balance of power on the ground" and help the Syrian people and rebels "exercise self-defence".
But Western powers among the Friends pointedly did not offer weapons, reflecting their alarm at the rise of radical Islamist groups like the Nusra Front in the insurgent ranks.
The rebel source said arms and ammunition would be delivered by Gulf Arab states like Saudi Arabia and Qatar while the United States and others would focus on training and non-lethal aid.
"Some just want to provide non-lethal stuff, which is fine. These things are also needed. Along with intelligence help and training," he said.
WEST DIFFERS OVER PRIORITIES AT MEETING
Two Western diplomats said the main purpose of Saturday's Istanbul meeting was not to approve arms shipments.
One diplomat said that among the main objectives would be to get clear commitments from the Syrian National Coalition to take a firm stance against extremism and to improve their unity and planning for the post-Assad era.
A second diplomat said the "meeting is not about arming the rebels - some people have leapt to conclusions, including the Syrians, but this meeting is not about weapons.
"The meeting is about 'more for more', 'promises for promises'. If the opposition can give more, then we can give more. We are talking about more political commitments, like an inclusive political vision, chemical weapons assurances, how to behave as a fighting force, how they see a Syria after Assad.
"After this, we can give more ambitious help, humanitarian and non-humanitarian aid, but not weapons. There are plenty of ways to support the armed opposition without giving weapons."
U.S. Defense Secretary Chuck Hagel said on Wednesday that Washington is dispatching army planners to Jordan as neighbouring Syria's conflict worsens but he signalled deep misgivings about direct U.S. military intervention.
Rebels say U.S. officers have been training in Jordan groups of moderate rebels mainly from Deraa and Damascus province in recent months on intelligence gathering and the use of arms.
Referring to the Jordan training, the rebel source said there was now a clear approach from the Friends of Syria to provide weapons to rebels who are part of the Syrian National Coalition's military arm.
"Things are changing now and the Americans are sending troops to Jordan, the way the international community was dealing with the regime has changed now. They also want actions and it is time to get rid of (Assad) for good," he said.
Turkish Foreign Minister Ahmet Davutoglu said late on Thursday that it was important to establish a humanitarian aid corridor in Syria. He cited a need for a clear and determined stance to provide assistance to those Syrians trying to stay alive under bombardment.
Syrian government forces had fired 205 Scud missiles from Damascus to Aleppo in the last few months, he said. "It is now time to act on war crimes. They must be held to account. Nobody could say these are not war crimes," Davutoglu said of the missile strikes in an interview with broadcaster CNN Turk.
George Sabra, deputy head of the Syrian National Coalition, told Reuters that the growth of extremism in Syria arose from a lack of action by world powers.
"(They) acknowledged our right to defend ourselves and when you acknowledge someone's right you need to provide him with what is commensurate with this right." (Additional reporting by Nick Tattersall in Istanbul and Jonathon Burch in Ankara; Writing by Oliver Holmes; Editing by Mark Heinrich)