By Louis Charbonneau

UNITED NATIONS (Reuters) - U.N. Secretary-General Ban Ki-moon on Monday told the Security Council that the international team of experts who will oversee the dismantling of Syria's chemical weapons team will number around 100 and will likely require help from U.N. member states.

An advance team of experts from the Organization for the Prohibition of Chemical Weapons with U.N. support personnel on Sunday oversaw Syrian forces using blowtorches and angle grinders to render missile warheads and bombs unusable. The demolition work is due to last until mid-2014.

The letter to the 15-nation council said the dismantling would take place in three phases - an initial phase, which has begun; a second phase involving the destruction of chemical weapons and mixing equipment; and a third phase in which the total elimination of the program will be verified and monitored.

"Phase III will be the most difficult and challenging phase," according to the letter, which was obtained by Reuters.

"From 1 November 2013 to 30 June 2014 ... the Joint Mission will be expected to support, monitor and verify the destruction of a complex chemical weapons program involving multiple sites spread over a country engulfed in violent conflict, which includes approximately 1,000 metric tons of chemical weapons, agents and precursors," Ban said in the letter.

He added that those substances are "dangerous to handle, dangerous to transport and dangerous to destroy."

"The Joint Mission will build upon the advance team deployment and expand to a staff of approximately 100 personnel from both the OPCW and the United Nations," Ban said.

He said that he would appoint a "special coordinator" to oversee the joint operation and liaise with the United Nations, The Hague-based OPCW and the Syrian government.

The mission will maintain a "light footprint," deploying only the personnel needed at any given time, and it will operate out of Cyprus and Damascus, Ban said.

While the Syrian government will provide security in the initial phases of the demolition program, Ban said that in the third and final phase it was "highly probable that assistance by other Member States will be required in the areas of the provision of both technical and operational advice, support and equipment, as well as security."

Russia, Syria's long-time ally and arms supplier, has offered to assist with the demolition process.

The Security Council will discuss Ban's recommendations later this week, council diplomats said.

Syrian President Bashar al-Assad's government, fighting a civil war in which more than 100,000 people have died according to U.N. figures, agreed to destroy the chemical weapons after a sarin gas attack on the outskirts of Damascus killed hundreds of people on August 21 and led to threats of U.S. airstrikes.

Assad's government and rebels trying to oust him blame each other for what was the world's worst chemical weapons attack in 25 years. Western governments say a U.N. investigation of the incident indirectly implicates forces loyal to Assad.

The Security Council adopted a resolution late last month that demands Syria abandon its chemical weapons program. Shortly before that, Assad's government acceded to the chemical weapons convention that ban's poison gas arsenals.

That resolution was based on a joint U.S.-Russia deal agreed last month in Geneva.

(Reporting by Louis Charbonneau; Editing by Paul Simao)