* Syria submits partial document to chemical arms body
* OPCW expected to review disarmament plan next week
* Rebels dismayed by U.S.-Russian deal as war goes on
* Syria breaking up into many rival fiefdoms
By Anthony Deutsch and Oliver Holmes
THE HAGUE/BEIRUT, Sept 20 (Reuters) - Syria gave details of some of its chemical weapons to a U.N.-backed arms watchdog at The Hague on Friday but needs to fill in gaps by next week to launch a rapid disarmament operation that may avert U.S. air strikes.
At the Organisation for the Prohibition of Chemical Weapons, the agency which is to oversee the removal of President Bashar al-Assad's arsenal, a spokeswoman said: "We have received part of the verification and we expect more."
She did not say what was missing from a document one U.N. diplomat described as "quite long". The OPCW'S 41-member Executive Council is due to meet early next week to review Syria's inventory and to agree on implementing last week's U.S.-Russian deal to eliminate the entire arsenal in nine months.
The timetable was laid down by U.S. Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov a week ago in Geneva when they set aside sharp differences over Syria to agree on a plan to deprive Assad of chemical weapons and so remove the immediate threat from Washington of launching military action.
That plan set a rough deadline of Saturday for Syria to give a full account of the weapons it possesses. Security experts say it has about 1,000 tonnes of mustard gas, VX and sarin - the nerve agent U.N. inspectors found after hundreds were killed by poison following missile strikes on rebel-held areas on Aug. 21.
Kerry said he had spoken to Lavrov by telephone on Friday and agreed to continue cooperating, "moving not only towards the adoption of the OPCW rules and regulations, but also a resolution that is firm and strong within the United Nations".
One Western diplomat warned that a failure by Assad to account for all the suspected stockpile would cause world powers to seek action at the U.N. Security Council to force him to.
The U.S. State Department said it was studying the material: "Today was a step that we're looking for in terms of an initial document," said spokeswoman Marie Harf. "We will be taking a look at it and making an assessment ... An accurate list is vital to ensure the effective implementation."
The United States and its allies said the U.N. inspectors' report this week left no doubt Assad's forces were responsible for the Aug. 21 killings. Assad, however, has blamed the rebels and Moscow says the evidence of responsibility is unclear.
The Syrian government has accepted the plan and has already sought to join the OPCW. For Assad, the Russian proposal to remove chemical weapons provided an unexpected reprieve from the military action which President Barack Obama had planned after the Aug. 21 attack. For Obama, it solved a dilemma posed when he found Congress unwilling to support war on Syria.
Once the OPCW executive has voted to follow the Lavrov-Kerry plan in a meeting expected early next week, the Security Council is due to give its endorsement of the arrangements - marking a rare consensus after two years of East-West deadlock over Syria.
However, Russia, which has a veto, remains opposed to attempts by Western powers to have the Security Council write in an explicit and immediate threat of penalties - under what are known as Chapter VII powers. It wants to discuss ways of forcing Syrian compliance only in the event Damascus fails to cooperate.
Obama has warned that he is still prepared to attack Syria, even without a U.N. mandate, if Assad reneges on the deal.
Syria's rebels, who have been fighting to end four decades of Assad family rule since 2011, have voiced dismay at the U.S.-Russian pact and accuse their Western allies of being sidetracked by the chemical weapons issue while Assad's forces use a large conventional arsenal to try to crush the revolt.
That may see the official opposition look more to its Arab and Turkish supporters for help.
It may also hamper Western - and Russian - efforts to bring the warring parties together for a peace conference. Moscow and Washington have said progress on removing chemical weapons could pave the way for a broader diplomatic effort to end a conflict that has killed well over 100,000 and destabilised the region.
The increasing bitterness of the fighting, especially along sectarian lines, and also a fragmentation into rival camps, particularly on the rebel side, will also hamper negotiations.
On Friday, al Qaeda-linked fighters and a unit of Syrian rebels declared a truce after two days of clashes in the town of Azaz near the Turkish frontier that highlighted divisions in the opposition, in which hard line groups are powerful.
Assad's army, backed by Shi'ite regional power Iran and dominated by officers from Assad's Alawite religious minority, has mobilised militia and fighters from the Lebanese Shi'ite militant group Hezbollah. Alawites are a Shi'ite offshoot.
Most rebels are from Syria's Sunni Muslim majority. But factions have split as foreign fighters driven by jihad have flocked to the country, often at odds with local Syrians. Ethnic Kurds in the north have fought both sides.
Fighters from al Qaeda affiliate the Islamic State of Iraq and the Levant, also known as ISIS, have battled the Northern Storm Brigade, a group that controls the border.
The Syrian National Coalition, political exiles who work with the Western-backed Free Syrian Army (FSA), accused the jihadist group of "aggression towards Syrian revolutionary forces and its indifference to the lives of the Syrian people".
"ISIS no longer fights the Assad regime. Rather, it is strengthening its positions in liberated areas, at the expense of the safety of civilians," it said in a statement, attacking the group for this week's fighting at Azaz.
In other parts of Syria, al Qaeda-affiliated forces have enticed rebels to join them. Hundreds of rebels, including entire brigades, have pledged allegiance to ISIS and its domestic branch the Nusra Front in northern and eastern Syria, activists and Islamist sources said on Friday.
Washington says the chemical weapons deal has restarted talk of a second peace conference in Geneva. The first round of peace talks in June 2012 failed to end hostilities, but its supporters say it created the framework for an eventual settlement.
Last year's Geneva agreement aimed to create a transitional government with full executive powers agreed by both the Damascus administration and the Syrian National Coalition (SNC).
But the plan leaves out major players on the ground whose role has grown since. Pro-Assad militias, Kurdish militant groups, al Qaeda-linked rebels and other Islamist brigades that do not pledge allegiance to the FSA are not part of the deal.
"Let's be clear on this, Geneva 2 will not stabilise Syria," said Lebanon-based political scientist Hilal Khashan. "It will open a new chapter in the Syria conflict."
He said that even if the SNC and the government agreed on a transition government, jihadist groups would continue to fight and Kurdish militants would seek autonomy.
Khawla Mattar, spokeswoman for U.N. Syrian envoy Lakhdar Brahimi, said the onus is on the SNC to be representative of Syrian society: "The Coalition ... have to bring the widest representation of Syrian society."
Aid agencies raised the alarm on Friday about the break up of Syria into pockets run by different factions.
At a Geneva news conference, Pierre Kraehenbuehl, director of operations for the International Committee of the Red Cross, said: "When colleagues of ours travel from Damascus to Aleppo, it is something between 50 and 60 checkpoints on the way."