Kofi Annan predicts 'appropriate outcome' for weekend talks, but difficulties remain over possible departure of Bashar al-Assad

Russia and the US remained locked in negotiation on Friday over proposals for a political solution to the Syrian crisis as violence and protests raged across the country on the eve of a key international conference.

Kofi Annan, envoy for the UN and Arab League, predicted an "appropriate outcome" for Saturday's talks in Geneva. But disagreements between Moscow and the west over the possible departure of President Bashar al-Assad and continuing carnage on the ground underlined how hard it will be to make progress.

Opposition activists reported government forces attacking the Damascus suburb of Douma, scene of what the Syrian Revolution Coordination Committee called a "brutal massacre" of 60 people. Video posted online showed corpses in white burial shrouds lined up in a street.

Helicopters were used to hit nearby Harasta and Barzeh while demonstrations calling for Assad's removal took place from Aleppo in the north to Daraa in the south. On Thursday alone 180 people were said to have been killed – making it one of the bloodiest days of the 16-month uprising.

In another significant development, the Free Syrian Army announced the capture of two senior security officers, Major General Faraj al-Maqt and Brigadier General Mounir Shleibi, reinforcing the view rebel operations are becoming more effective.

US secretary of state Hillary Clinton was meeting her Russian counterpart, Sergei Lavrov, in St Petersburg to discuss disagreements as Moscow demanded changes to a draft document submitted by Annan to which it had previously privately agreed.

The paper on "guidelines and principles" cites no names but implies Assad and close aides would have to step down. "A transitional unity government would have to exclude those whose continued presence and participation would undermine the credibility of the transition and jeopardise stability and reconciliation," it says.

Such tortuous language matters because it acknowledges Russia's refusal to back what it calls "regime change" while publicly supporting the part of Annan's six-point peace plan that calls for a "Syrian-led political process". Preparatory talks in Geneva on Friday stalled over Moscow's insistence on excising the reference to exclusion.

"Is this the Russians reneging on something they have already agreed to or just improving their negotiating position?" asked one official. "It's not entirely clear."

Syrian opposition groups insist there can be no agreement on transition without Assad surrendering power and leaving the country.

According to the document, seen by the Guardian, the establishment of a transitional government would be followed by a national dialogue and a review of Syria's constitutional order and the legal system. Next would come "free and fair multi-party elections for the new institutions and offices that have been established".

It also urges a "cessation of armed violence in all its forms and immediate, credible and visible actions to implement items two to six of Annan's six-point plan". These include the withdrawal of troops from cities, humanitarian and media access, and the release of prisoners.

The invitees to Saturday's Geneva talks are the foreign ministers of the five permanent members of the UN security council as well as Turkey and three Arab states. Iran and Saudi Arabia, key but controversial players, have not been asked.

The Annan document also addresses the sensitive question of continuity of Syrian governmental institutions and staff. "The public services must be preserved or restored. This includes the military forces and security services. However, all governmental institutions, including the intelligence services, have to perform according to human rights and professional standards and operate under a top leadership that inspires public confidence."

The document adds, without elaboration, that "accountability for acts committed during the present conflict must be addressed". It calls too for a comprehensive package for transitional justice, including compensation or rehabilitation for victims of the present conflict, steps towards national reconciliation and forgiveness."

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