By Stephanie Nebehay

GENEVA (Reuters) - The Syrian opposition and a senior Russian diplomat agreed on Wednesday that Syria's fragile ceasefire needed to be shored up amid growing violations in the multi-sided civil war.

Preserving the cessation of hostilities in place between the Russian-backed Syrian government and rebels since Dec. 30 is seen as key to any hope of progress in Geneva peace talks mediated by U.N. Special Envoy Staffan de Mistura.

But Western diplomats were skeptical about the Geneva talks.

"We are in a game of just keeping the whole thing rolling with no real sign of a substantive breakthrough at all," a Western envoy told Reuters. "We are not looking at any real negotiations here."

In the latest bloodshed, a bomb blast hit a passenger bus in the government-held city of Homs on Wednesday, killing five people and wounding six, the Syrian state news agency SANA reported.

After meeting opposition negotiators, Russian deputy foreign minister Gennady Gatilov said the truce, which is backed by Russia, Iran and Turkey, was "more or less being maintained", but major powers needed to do more to extend it.

"Here, we need the support and involvement of all parties, including external players, not only the three guarantors but some other countries who also have influence on the parties on the ground," Gatilov told reporters.

Opposition delegates told Gatilov that they had come to Geneva to engage "completely and seriously" in the political talks and had made steps forward on the U.N. peace agenda, opposition spokesman Salem al-Muslet told reporters.

The U.N. agenda includes constitutional change, political transition, better governance and U.N.-sponsored free elections.

"However the other (Syrian) party has taken no steps in this regard," Muslet said. "We discussed the ceasefire and continuous violations by the regime in addition to the Iranian presence in Syria and the forced displacement taking place in several areas."

The Shi'ite Muslim residents of two pro-government towns under rebel siege are to be evacuated in exchange for the evacuation of Sunni Muslim rebels and their families from two government-besieged towns in a mediated deal, according to a pro-government source and the Syrian Observatory for Human Rights monitoring group.

Russia, President Bashar al-Assad's main ally, plans to hold the next round of ceasefire talks in the Kazakh city of Astana in the first week of May, Muslet said, suggesting that the rebels might boycott.

"We have seen a lot of violations including the displacement in (Wadi) Barada or al-Waer, or other areas in addition to the ongoing bombardment (of) the (rebel-controlled) countryside of Damascus. So this will be the real test for the guarantors for the ceasefire to take place so we can participate."

(Additional reporting by Marina Depetris; writing by Stephanie Nebehay; editing by Tom Miles and Mark Heinrich)