* Crisis envoy to stay in Syria until Sunday for talks
* Pope calls for humanitarian aid, prays for peace in Syria
* Opposition activists vow rebel goodwill toward Christians
By Erika Solomon
BEIRUT, Dec 25 (Reuters) - International envoy Lakhdar Brahimi pursued mediation efforts in Damascus on Tuesday, but there was no pause in the bloodletting as Syrian Christians marked a bleak Christmas Day with prayers for peace.
"We are here in a cave that symbolises Syria right now," said a priest standing beside a nativity scene in a grotto.
"It is cold here but the door is open to all refugees," he told Syrian state TV. "Amid the hunger, cold and deprivation, we still have hope for peace and love for our country."
More than 44,000 Syrians have been killed since a revolt against President Bashar al-Assad erupted 21 months ago, igniting an increasingly sectarian conflict that broadly pits a Sunni Muslim majority against Assad's Alawite minority.
Christians, many of whom have been reluctant to join what they see as an Islamist-tinged insurgency, feel threatened.
Bishop John Kawak, speaking on state TV, said the Christmas holiday was "a symbol for the rebirth of the nation". He condemned "terrorism", the government's term for the rebellion.
Brahimi met some dissidents who are tolerated by Assad but rejected by the mainstream opposition and by rebels fighting to oust him, a day after he held talks with the Syrian president.
There was no word on any progress in the U.N.-Arab League' envoy's drive to end violence that has intensified in recent months as Assad uses airpower and artillery against rebel gains.
Raja Naser, secretary general of the National Coordination Body, said after meeting Brahimi that the envoy planned a week of meetings in Damascus and would stay until Sunday.
"There is still a lot of concern but there is also great hope that these meetings with other Syrian officials will result in some agreements or positive developments," he said.
But most opposition groups appear frustrated with Brahimi's quest for a deal on a transitional government. He has not clarified any role for Assad, whose foes say he must simply go, arguing that too much blood has been shed for any other outcome.
Gulf Arab leaders, who have long called for Assad's removal and some of whom have helped the rebels with guns and money, urged swift world action to halt the "massacres" and violations of international law in Syria.
The pro-opposition Syrian Observatory for Human Rights reported clashes and government shelling in hotspots across the country, including towns on the eastern outskirts of Damascus.
Abu Nidal, a spokesman for the Rebel Military Council in Damascus, said fighters had killed the head of a local security branch in the capital's suburb of Jaramana, home to a large Christian and Druze population.
In his Christmas message to the world on Tuesday, Pope Benedict encouraged Syrians not to lose hope for peace.
"May peace spring up for the people of Syria, deeply wounded and divided by a conflict which does not spare even the defenceless and reaps innocent victims," he said.
"I appeal for an end to the bloodshed, easier access for the relief of refugees and the displaced, and dialogue in the pursuit of a political solution to the conflict."
Hundreds of thousands of Syrians have fled abroad to escape the daily violence. Those who remain face severe shortages of food, fuel and other essentials as winter weather takes a grip.
Syrian activists offered a message of solidarity with Christians despite rising tensions in central Hama province, where rebels have demanded that Christian villages let them enter to force out the army and pro-Assad "shabbiha" militias.
"We say to the Christians, you are our brothers and our beloved, and your holiday is our holiday," said Abu Faisal, a Hama activist who posted a Christmas message on the Internet.
"The rebels are surrounding (the Christian town) Muhardeh to get rid of Assad's soldiers and shabbiha, but we have not forgotten your honourable stance when you took care of our refugees when the army entered Hama," he said.
"We will not accept that you are targeted by hatred, you are our brothers and our friends."