MOSCOW, June 28 (Reuters) - Multilateral talks on U.N. envoy Kofi Annan’s Syrian mediation plan should seek to secure a ceasefire but not determine in advance the shape of a possible government of national unity, Russia said on Thursday.

“The meeting in Geneva was intended to support Kofi Annan’s plan and it must set the conditions for the end of violence and the start of an all-Syrian national dialogue, and not predetermine the contents of this dialogue,” Foreign Minister Sergei Lavrov told a briefing.

Lavrov is expected to discuss Annan’s proposal, aimed at ending the 16-month conflict in Syria, with the other four permanent U.N. Security Council members and key players in the Middle East in Geneva on Saturday.

Russia and other big powers have told Annan that they support his idea of a Syrian national unity cabinet that could include government and opposition members but would exclude those whose participation would undermine it.

The idea of excluding certain people was seen by diplomats as referring to Syrian President Bashar al-Assad, although Annan’s proposal did not explicitly say the Syrian leader could not serve in a national unity government.

Moscow has backed Annan’s peace plan, insisting it is the only way to end the bloodshed in Syria and arguing firmly against any kind of military intervention.

Speaking in Moscow, Lavrov said that the Annan plan was not, however, a final document and he expressed dismay that it had been leaked to the media ahead of the Geneva talks.

Lavrov reiterated Russian warnings against a Libyan-style intervention in Syria, warning the consequences would be “more catastrophic”.

He also criticised the exclusion of regional power Iran from the negotiations. Annan has said that Iran should attend the Geneva talks but diplomats say that the United States, Saudi Arabia and other countries objected. (Reporting By Nastassia Astrasheuskaya; Writing by Alissa de Carbonnel; Editing by Douglas Busvine and Jon Boyle)