* About 40,000 people killed during 20-month Syria conflict
* Brahimi wants resolution based on world powers Geneva deal
* U.N. Security Council deadlocked over stronger action
By Michelle Nichols
UNITED NATIONS, Nov 29 (Reuters) - International Syria mediator Lakhdar Brahimi said on Thursday he could not move forward with a peace plan unless it was backed by a U.N. Security Council resolution and he warned that a ceasefire would only hold if it was overseen by a peacekeeping mission.
A peaceful solution to the 20-month conflict could only be initiated by the Security Council, Brahimi told the 15-member body, which has been deadlocked on whether to take stronger action against Syrian President Bashir al-Assad.
"I have been challenged to produce a plan. I think that we now have the elements of a plan, but these elements cannot be put together until this council has come together and is ready to adopt a resolution that will be the basis for a political process (in Syria)," Brahimi told reporters after his briefing.
The United States and European council members blame Russia, a staunch ally and key arms supplier for Assad's government, and China for the council's inaction on the conflict. Moscow and Beijing have vetoed three resolutions condemning Assad and reject the idea of sanctioning his government.
"No doubt what is very, very urgently needed is a ceasefire that can hold ... a ceasefire will not hold unless it is very, very strongly observed, and that, I believe, will require a peacekeeping mission," Brahimi said.
If there were a more sustained ceasefire, the U.N. Department of Peacekeeping Operations has told Brahimi it could put together a force of up to 3,000 monitors to keep fighters separated and maintain the truce, diplomats say.
Under a truce in April brokered by Brahimi's predecessor, former U.N. Secretary-General Kofi Annan, some 300 monitors and about 100 civilian experts were deployed to Syria to oversee the ceasefire that failed to take hold. As the conflict worsened, the Security Council allowed the force's mandate to lapse and the unarmed monitoring team withdrew in August.
A Syrian revolt against Assad began as peaceful rallies calling for more freedoms and democracy, but it turned into an armed struggle after the military cracked down on protesters. It has killed about 40,000 people, making it the bloodiest of Arab uprisings that have ousted entrenched leaders in Tunisia, Egypt, Libya and Yemen since early last year.
Brahimi wants the Security Council to pass a resolution based on a deal reached by world powers in June to set up a transitional government in a bid to end the bloodshed. "The new Syria will not look like the Syria of today," he said.
Russian U.N. Ambassador Vitaly Churkin said Moscow vetoed an attempt this year to back the deal with a resolution because it had been rewritten to be become an "instrument of further confrontation and building up tensions in Syria, putting more pressure on the Syrian government."
He said Russia was prepared to hold consultations on a resolution to bring about a political solution because "a military solution is not really working." But he again ruled out sanctions against Assad's government.
"Some say 'well, if we threaten more sanctions to the government, then they will do something about it.' We do not really buy that," Churkin told reporters. "There is as much pressure as one can think of in terms of where the Syrian government finds itself right now."
"We need really to find responsible people on both sides who need to realize that its time for them ... to enter into dialogue. Incidentally, we are not saying that President Assad should be sitting at the table," he said.
Syrian U.N. Ambassador Bashir Ja'afari again accused "foreign hands interfering in the domestic affairs of my country Syria" and called for the violence to stop. (Reporting by Michelle Nichols, editing by Stacey Joyce)