* Mediator meets sides separately, urges talks on core issues
* Government side says combating "terrorism" must come first
* Attack on aid convoy in Homs, Islamist offensive loom large
A second round of Syria peace talks got off to a shaky start on Monday with the international mediator meeting the two sides separately after violations of a local ceasefire and an Islamist offensive set back his efforts.
Ahead of the talks, mediator Lakhdar Brahimi told delegates to commit first to discussing both ending the fighting and setting up a transitional government. The government side said combating "terrorism" - its catchall term for the revolt - should be agreed first.
The second round was intended to follow quickly after no substantive progress was made last month at the first round of talks in nearly three years of civil war.
Brahimi had tried to break down mutual distrust by focusing on agreeing a truce for a single city, Homs, but even that was only achieved after the first round was over, and aid workers were fired upon as they evacuated civilians on Saturday.
A letter from Brahimi given to the delegates over the weekend said the new round aimed to tackle the issues of stopping violence, setting up a transitional governing body, and plans for national institutions and reconciliation.
The opposition says a transitional governing body must exclude President Bashar al-Assad. The government says it will not discuss his leaving power.
The opposition said it had handed Brahimi its view of what a transitional government should look like and submitted witness statements it said showed the Syrian army had fired at the Homs aid convoy. The government blames the rebels.
The opposition also said there had been an escalation in the government's use of "barrel bombs" - oil drums or cylinders packed with explosives and metal fragments and usually dropped from helicopters. It said more than 1,800 Syrians had been killed by them last week, half in rebel held parts of Aleppo.
"It is not acceptable that the regime will send its own delegation to peace talks while it is killing our people in Syria. This must stop," opposition spokesman Louay al-Safi told reporters after the delegation met Brahimi.
SYRIA SAYS "CONSTRUCTIVE"
The mediator plans to keep meeting the two sides separately in Geneva over the next few days in hopes of improving the atmosphere at the talks, which are expected to last a week.
The Syrian government delegation urged him to condemn an Islamist offensive on Sunday which it said killed at least 41 people in the town of Maan in central Syria populated mainly by Assad's Alawite sect, a source said.
"Ending violence and combating terrorism and requiring the countries supporting (terrorism) to stop ... is the first issue that should be agreed upon to pave the way for the launch of the political process," the government delegation said in a document seen by Reuters which a source said it gave Brahimi on Monday.
Islamist fighters, who are not represented at the talks, seized a village in the central province of Hama on Sunday, part of an offensive to try to cut off supply routes from Damascus to the north of the country.
The Syrian Observatory for Human Rights monitoring group said the Islamists killed 25 people in the village of Maan, mainly from a pro-Assad National Defence Force militia.
The government said the dead were mainly women and children and accused the fighters of committing a massacre on the eve of the resumption of peace talks in Geneva.
Russia, Assad's main backer on the United Nations Security Council, reiterated its view that the opposition delegation was not representative enough of the forces on the ground.
Powerful Islamist groups are boycotting the talks and have denounced the opposition team, made up mainly of political exiles, as traitors.
AL QAEDA INFIGHTING
Activists and rebels said an al Qaeda splinter group had withdrawn its forces from Syria's oil-rich eastern province of Deir al-Zor after days of heavy fighting with its rivals.
Islamist opposition groups joined forces with some secular rebel units to fight the Islamic State of Iraq and the Levant (ISIL), with whom they have territorial disputes and ideological differences.
ISIL, which has attracted many foreign Islamist militants into its ranks, is a small but powerful fighting force in Syria's opposition areas. It has alienated many civilians and opposition activists, however, by imposing harsh rulings against dissent in areas it controls, such as beheadings.
The civil war which developed after a crackdown on protests against Assad has killed more than 100,000 people, forced millions to flee their homes, and is destabilising neighbouring countries.
One Western diplomat said the aid convoy attack was a bad sign and the government was showing little sign of commitment to the talks. "Homs is not encouraging. They (the government) are not making a colossal effort," the diplomat said on Monday.
Syrian Deputy Foreign Minister Faisal Mekdad said the government was not trying to impose a new agenda.
"We want to discuss the Geneva Communiqué point by point, until the end of this list. This is the Syrian approach to this discussion. I think this is a very, very constructive approach that should not create any problems."