* Brahimi says outside help needed to get sides talking
* Plan stalled on rebel demands to exclude Assad
* Government forces seize Homs district
* Reports of "massacre" cannot be verified
By Yara Bayoumy and Maria Golovnina
AZAZ, Syria/CAIRO, Dec 30 (Reuters) - The international peace negotiator for Syria pleaded with outside countries on Sunday to push the warring parties to the table for talks, warning that the country would become a failed state ruled by warlords unless diplomacy is given a chance.
Lakhdar Brahimi, who inherited the seemingly impossible task of bringing an end to the war after his predecessor Kofi Annan resigned in frustration in July, has launched an intensified diplomatic campaign to win backing for a peace plan.
He spent five days this week in Damascus, where he met President Bashar al-Assad. On Saturday he visited Assad's main international backers in Moscow, and on Sunday he travelled to Cairo, where President Mohamed Mursi has emerged as one of Assad's most vocal Arab opponents.
"The problem is that both sides aren't speaking to one another," he said. "This is where help is needed from outside."
Brahimi's peace plan - inherited from Annan and agreed to in principle in Geneva in June by countries that both oppose and support Assad - has the seemingly fatal flaw of making no mention of whether Assad would leave power.
The Syrian leader's opponents - who have seized much of the north and east of the country in the past six months - say they will not cease fire or join any talks unless Assad goes and have largely dismissed Brahimi's initiative.
But Brahimi says the plan is the only one on the table, and predicts "hell" if countries do not push both sides to talk.
"The situation in Syria is bad, very, very bad, and it is getting worse, and the pace of deterioration is increasing," Brahimi told reporters.
"People are talking about Syria being split into a number of small states ... This is not what will happen. What will happen is Somalisation: warlords." Somalia has been without effective central government since civil war broke out there in 1991.
More than 45,000 people have been killed in Syria's 21-month war, the longest and deadliest of the revolts that began sweeping the Arab world two years ago.
The rebels are mainly from the Sunni Muslim majority, fighting against Assad, a member of the Shi'ite-derived Alawite minority sect, giving the war a dangerous sectarian dimension.
The rebels increasingly believe that their military successes of the past half year are bringing victory within reach. But Assad's forces still hold the densely-populated southwest of the country, the main north-south highway and the Mediterranean coast in the northwest.
The government also holds airbases scattered throughout the country, and has an arsenal including jets, helicopters, missiles and artillery that the fighters cannot match.
Government troops scored a victory on Saturday after several days of fighting, seizing a Sunni district in Homs, a central town that controls the vital road linking Damascus to the coast.
Opposition activists said on Sunday that many people had been killed in the Deir Baalbeh district after it was captured, although it was not immediately possible to verify claims that a "massacre" had taken place. The opposition Syrian Network for Human Rights said it documented the summary execution of 17 men.
"They were young and old, mostly refugees who had fled to Deir Baalbeh from central parts of Homs," it said in a statement. Footage taken by activists showed the bodies of eight men with what appeared to be bullet wounds in the face and head.
With severe restrictions by Syrian authorities on independent media in place since the revolt broke out in March last year, the footage could not be confirmed.
Najati Tayyara, a veteran opposition campaigner from Homs in contact with the city, told Reuters residents believed the death toll was as high as 260, although the area was sealed off by government forces and allied militia.
"I am afraid that we have seen a massacre in Deir Baalbeh and a military setback for the rebels because of their lack of organisation. They have been in need of ammunition for a long time and it finally ran out," he said.
"Communications are difficult and we are trying to piece together what happened in Deir Baalbeh. We so far know that regime forces went in after the rebels retreated and summarily executed dozens of people, including civilians."
Tayyara said the fall of Deir Baalbeh undermined supply lines to rebel held areas inside the city.
Bilal al-Homsi, an opposition activist in Old Homs, said MiG warplanes bombarded the area overnight and medium range rockets and hit the area of al-Khalidiya, a rebel-held Sunni district.
In the north, opposition activists said fighters had surrounded an air defence base near Aleppo airport, south of the contested city. Fighting raged in the area and warplanes bombed rebel positions near the base to try and break the siege.
In the northern city of Azaz, where activists said 11 people were killed when air strikes destroyed six homes, gravediggers were already digging graves for whichever victims will be next.
"We know the plane is coming to hit us, so we're being prepared," said Abu Sulaiman, one of a few men digging at the Sheikh Saad cemetery.
"Massacres are happening. We're putting every two or three bodies together. We've been working and digging since 6 in the morning. We're going to dig 10 new graves today," he said.
"We're preparing them. Maybe we'll be buried in them."
Fida, a 15-year-old girl in a green scarf and purple coat looked on as her father shovelled dirt from the gravesite. The dead from the previous day's attacks included friends she recognised when their shrouds were pulled back.
"Yesterday was the first time I uncovered blankets to discover that my friends had died," she said, as young children near the cemetery played hopscotch on the streets and kicked stones about.
"I was just about to go visit them about a half hour before the strike hit," she said. "In the end I visited them when they were dead."