Damascus (AFP) - Syria's regime agreed Tuesday to a ceasefire deal announced by the United States and Russia after it was conditionally accepted by the opposition but doubts persisted it could take effect by the weekend.

The truce agreement, announced Monday, does not apply to jihadists like the Islamic State group and Al-Nusra Front, putting up major hurdles to how it can be implemented on Syria's complex battlefield.

A foreign ministry statement said the Syrian government would continue to fight both those groups as well as other "terrorists", while agreeing to stop other military operations "in accordance with the Russian-American announcement."

The deal calls for a "cessation of hostilities" between forces loyal to President Bashar al-Assad and opposition groups that would take effect at midnight Friday Damascus time.

The High Negotiations Committee (HNC) -- the leading Syrian opposition group -- gave its conditional acceptance to the deal late Monday.

But after several previous failed attempts, few had serious expectations for a lasting ceasefire.

Analysts said the deal may be simply unworkable, rebels on the ground doubted the regime's goodwill and many civilians expected their hopes to once again be dashed.

"It's a waste of time and it's difficult to implement on the ground," said Abu Ibrahim, a commander in the 10th Brigade opposition force in the northwestern Latakia province.

He expected "numerous rebel groups" to reject the agreement, which he said was formed "without consulting any factions on the ground."

- Truce deal 'likely doomed' -

French President Francois Hollande said Tuesday that the ceasefire must be implemented, after discussing it with Britain, Germany and the United States.

The truce "must be fully respected and the sooner, the better," Hollande told reporters during a visit to Peru.

But in Damascus, residents tired after nearly five years of war were sceptical over the deal.

"It's a fragile deal," said Rana, a 54-year old pharmacist in the capital.

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"Ceasefires have been announced repeatedly in the past and we didn't see any results on the ground because they were violated," she said.

Despite being on opposing sides of the conflict, Moscow and Washington have been leading the latest diplomatic push to try to resolve the brutal conflict.

Both powers are pursuing separate air wars in Syria, with a US-led coalition targeting IS and occasionally other jihadist groups.

Russia says it is targeting "terrorists" in its strikes but has been accused of hitting non-jihadist groups in support of Assad, a longtime ally.

Analysts say that given the facts on the ground -- in particular the complicated make-up of Syria's opposition forces and frequently shifting frontlines -- the ceasefire may be doomed to fail.

While IS control over territory is relatively clear and stable, its jihadist rival Al-Nusra Front, the local affiliate of Al-Qaeda, works closely with many other rebels groups particularly in Syria's north.

"'Cessation of Hostilities' allows attacks on Nusra. That likely dooms it, since Russia/regime tend to hit others & call em Nusra (or IS)," Noah Bonsey, a senior analyst at the International Crisis Group, said on Twitter.

- Hope for peace talks -

The ceasefire plan was announced by top diplomats in Munich earlier this month, but failed to take hold last week as initially planned.

As well as reducing violence and expanding humanitarian access, it aims to pave the way for a resumption of peace talks that collapsed earlier this month in Geneva.

The talks had been scheduled to resume on February 25, but the UN Syria envoy has already acknowledged that date is no longer realistic.

US Secretary of State John Kerry said Tuesday that Washington was considering a "Plan B" if Damascus and Moscow are not serious about negotiating a political transition in Syria.

Briefing US lawmakers, Kerry said he had told Russia's President Vladimir Putin the United States would not wait more than a few months to see whether Assad is serious about talks.

But he would not be drawn on details of any "Plan B" that he would advise President Barack Obama to adopt if efforts to mediate a political deal to end the war fail.

Turkish Deputy Prime Minister Numan Kurtulmus told reporters in Ankara that he was "not very optimistic (the truce) will be respected by all the parties."

In a new toll released Tuesday, the Syrian Observatory for Human Rights monitoring group said that 271,138 people -- including nearly 80,000 civilians -- had lost their lives in the war.

Sunday saw the deadliest jihadist attack in Syria's conflict, when 134 people were killed in a Shiite Muslim shrine south of Damascus in bombings claimed by IS.

Also on Tuesday, IS and allied jihadists wrested control of the key town of Khanasser in the northern province of Aleppo from regime forces, the Observatory said.