* Syria's deputy foreign minister heads to Moscow
* Lebanese pro-Syrian source talks of optimism in Damascus
* Wary opposition says no deal can leave Assad in power
* Artillery attack kills 20 in northern village
BEIRUT, Dec 26 (Reuters) - Syrian President Bashar al-Assad sent a senior diplomat to Moscow on Wednesday to discuss proposals to end the conflict convulsing his country made by international envoy Lakhdar Brahimi, Syrian and Lebanese sources said.
Brahimi, who saw Assad on Monday and is planning to hold a series of meetings with Syrian officials and dissidents in Damascus this week, is trying to broker a peaceful transfer of power, but has disclosed little about how this might be done.
More than 44,000 Syrians have been killed in a revolt against four decades of Assad family rule, a conflict that began with peaceful protests but which has descended into civil war.
Past peace efforts have floundered, with world powers divided over what has become an increasingly sectarian struggle between mostly Sunni Muslim rebels and Assad's security forces, drawn primarily from his Shi'ite-rooted Alawite minority.
Deputy Foreign Minister Faisal Makdad flew to Moscow to discuss the details of the talks with Brahimi, said a Syrian security source, who would not say if a deal was in the works.
However, a Lebanese official close to Damascus said Makdad had been sent to seek Russian advice on a possible agreement.
He said Syrian officials were upbeat after talks with Brahimi, the U.N.-Arab League envoy, who met Foreign Minister Walid Moualem on Tuesday a day after his session with Assad, but who has not outlined his ideas in public.
"There is a new mood now and something good is happening," the official said, asking not to be named. He gave no details.
Russia, which has given Assad diplomatic and military aid to help him weather the 21-month-old uprising, has said it is not protecting him, but has fiercely criticised any foreign backing for rebels and, with China, has blocked U.N. Security Council action on Syria.
"ASSAD CANNOT STAY"
A Russian Foreign Ministry source said Makdad and an aide would meet Foreign Minister Sergei Lavrov and Mikhail Bogdanov, the Kremlin's special envoy for Middle East affairs, on Thursday, but did not disclose the nature of the talks.
On Saturday, Lavrov said Syria's civil war had reached a stalemate, saying international efforts to get Assad to quit would fail. Bogdanov had earlier acknowledged that Syrian rebels were gaining ground and might win.
Given the scale of the bloodshed and destruction, Assad's opponents insist the Syrian president must go.
Moaz Alkhatib, head of the internationally-recognised Syrian National Coalition opposition, has criticised any notion of a transitional government in which Assad would stay on as a figurehead president stripped of real powers.
Comments on Alkhatib's Facebook page on Monday suggested that the opposition believed this was one of Brahimi's ideas.
"The government and its president cannot stay in power, with or without their powers," Alkhatib wrote, saying his Coalition had told Brahimi it rejected any such solution.
While Brahimi was working to bridge the vast gaps between Assad and his foes, fighting raged across the country and a senior Syrian military officer defected to the rebels.
Syrian army shelling killed about 20 people, at least eight of them children, in the northern province of Raqqa, a video posted by opposition campaigners showed.
The video, published by the Syrian Observatory for Human Rights, showed rows of blood-stained bodies laid out on blankets. The sound of crying relatives could be heard in the background.
The shelling hit the province's al-Qahtania village, but it was unclear when the attack had occurred.
Rebels relaunched their assault on the Wadi Deif military base in the northwestern province of Idlib, in a battle for a major army compound and fuel storage and distribution point.
Activist Ahmed Kaddour said rebels were firing mortars and had attacked the base with a vehicle rigged with explosives.
The British-based Observatory, which uses a network of contacts in Syria to monitor the conflict, said a rebel commander was among several people killed in Wednesday's fighting, which it said was among the heaviest for months.
The military used artillery and air strikes to try to hold back rebels assaulting Wadi Deif and the town of Morek in Hama province further south. In one air raid, several rockets fell near a field hospital in the town of Saraqeb, in Idlib province, wounding several people, the Observatory said.
As violence has intensified in recent weeks, daily death tolls have climbed. The Observatory reported at least 190 had been killed across the country on Tuesday alone.
The head of Syria's military police changed sides and declared allegiance to the anti-Assad revolt.
"I am General Abdelaziz Jassim al-Shalal, head of the military police. I have defected because of the deviation of the army from its primary duty of protecting the country and its transformation into gangs of killing and destruction," the officer said in a video published on YouTube.
A Syrian security source confirmed the defection, but said Shalal was near retirement and had only defected to "play hero".
Syrian Interior Minister Mohammed Ibrahim al-Shaar left Lebanon for Damascus after being treated in Beirut for wounds sustained in a rebel bomb attack this month.