* Lakhdar Brahimi still weighing Syria envoy job-source
* France says Syria envoy job an 'impossible mission'
* U.N. Security Council to be briefed on Syria on Thursday
By Michelle Nichols and Mirna Sleiman
UNITED NATIONS/DUBAI, Aug 15 (Reuters) - Veteran diplomat Lakhdar Brahimi is not prepared to persist with the same "failed approach" on Syria as Kofi Annan, said a source close to the situation on Wednesday, and he is still considering whether to take on what France dubbed an "impossible mission."
Brahimi has been asked by the United Nations and the Arab League to replace Annan as their mediator on Syria at the end of the month, but diplomats say Brahimi has reservations and wants "strong support" from a deadlocked U.N. Security Council.
"It's an ... impossible mission, so I do understand that people hesitate to take this (on)," French U.N. Ambassador Gerard Araud, who is the Security Council president for the month of August, told reporters.
Annan said he was stepping down after six months in the job because he was hampered by the Security Council's hopelessly divided veto powers. Russia and China have vetoed three western-backed resolutions criticizing and threatening sanctions against Syrian President Bashar al-Assad's government.
It was not immediately clear what "strong support" Brahimi wanted from the 15-member council. Several Security Council diplomats said he had not made any specific requests regarding the form of support he wanted.
The source close to the situation said Brahimi wanted an agreement between the council's permanent veto powers - China, Russia, the United States, France and Britain - on Syria, but Security Council diplomats said such a deal was highly unlikely.
"Many want him (Brahimi) to just step into Kofi's shoes and just go on and either persist with the same failed approach or invent something unlikely," the source, who declined to be identified, told Reuters.
"Even though he is pressured on all sides to accept, he still has not accepted pending a number of basic conditions," the source said. "His conditions are the minimum for such a mission to go on."
An Arab League source, who declined to be identified, said Brahimi believed Annan failed because he lacked "crucial" U.N. Security Council support. But he added: "It is not clear to us what Brahimi wants exactly." Another diplomat in New York echoed the source's comments, saying Brahimi's demands were "vague."
Other candidates to replace Annan, envoys said, include former European Union foreign policy chief Javier Solana of Spain, former Spanish Foreign Minister Miguel Angel Moratinos, former Jordanian Foreign Minister Abdelilah al-Khatib, and Italian-Swedish diplomat Staffan de Mistura.
Annan's plan to end Syria's 17-month conflict - which calls for an end to violence, a Syrian-led political process, access for aid, the release of arbitrarily detained people, freedom of movement for journalists and the freedom to protest peacefully - failed to take hold, despite commitments from both sides.
The United States, which has stepped up non-lethal support to the rebels, saw little point in replacing Annan, given Russia's staunch opposition to sanctions, diplomats said. Qatar and Saudi Arabia are arming the rebels, diplomats say, and voiced little support for Annan's peace efforts.
U.N. Secretary-General Ban Ki-moon said on Thursday that more than 18,000 have been killed in the conflict in Syria, which began in March 2011 with a government crackdown on pro-democracy protesters.
"The Syrian people have suffered too much too long. We cannot go on this way. The international community must feel the sense of collective responsibility on this situation," Ban told a news conference while visiting East Timor.
The U.N. Security Council is due to be briefed on the situation in Syria on Thursday.
It is expected to allow a U.N. observer mission in Syria to expire on Sunday because violence has not receded enough for it to function properly. Russia has called for the monitors to remain, but the United States has made its opposition clear.
The mission's initial 300 unarmed observers, whose role has been to monitor a failed April 12 ceasefire, suspended their activity on June 16 because of increased risk from rising violence. There are also over 70 civilian staff working on a political solution and monitoring human rights violations.
U.N. diplomats have said that if the mission is allowed to expire, Ban would not need a new resolution from the deadlocked Security Council in order to maintain a political and humanitarian presence in the country.