Syrian President Bashar al-Assad has accepted a ceasefire deal agreed by Russia and the United States that is set to begin later this week, the state-run SANA news agency reported Tuesday.

The agreement is due to begin Saturday at midnight local time (2200 GMT Friday) but does not apply to Islamic State or other UN-designated terrorist organizations like the al-Nusra Front, al-Qaeda's Syrian branch.

Al-Assad, whose forces have been seizing territory from rebels in recent months thanks to Russian airstrikes, also excluded "terrorist" groups linked to those organizations.

The Syrian opposition, whose ground forces have been at a disadvantage, has been more cautious. Opposition activists have expressed doubts about the deal, questioning whether Russia would continue to launch airstrikes against rebels.

The Higher Negotiations Committee of rebel and opposition groups said its agreement is conditional on international guarantees that Russian and Iranian forces will respect the truce.

The opposition demanded the implementation of humanitarian measures, including an end to sieges, artillery shelling and air raids.

The Syrian government said it reserves the right to retaliate against any aggression on its positions, SANA reported, citing the Foreign Ministry.

Turkey, meanwhile, said it must be able to retaliate against the Kurdish People's Protection Units (YPG) militia in Syria if the group attacks Turkish territory.

"Turkey has the right to defend its territory," Numan Kurtulmus, the deputy prime minister, told reporters.

Turkey is generally in favour of the ceasefire deal, Kurtulmus said, but worries about potential Russian airstrikes against rebels.

The YPG has denied ever launching attacks against Turkish territory and says Turkey shelled its positions heavily last week. Turkey has not presented evidence of cross-border YPG attacks.

As part of the agreement between Moscow, an al-Assad ally, and Washington, which backs the YPG and, more cautiously, some rebel forces, the warring factions must indicate their willingness to abide by the ceasefire agreement by noon Friday.

The US acknowledged implementation will be difficult. "We know that there are a lot of obstacles, and there are sure to be some setbacks," White House spokesman Josh Earnest said.

Under the agreement, the US and Russia will exchange information to prevent parties to the ceasefire from being attacked by the US-led anti-Islamic State coalition, the Russian military, the Syrian military or opposition groups.

Airstrikes against Islamic State targets will continue and the sides will share information about territory controlled by designated terrorist groups.

In a video conference late Tuesday, US President Obama, British Prime Minister David Cameron, French President Francois Hollande and German Chancellor Angela Merkel "welcomed the prospect of a cessation of hostilities in Syria" as first on February 11, the White House said.

The Western leaders called for "an immediate halt to the indiscriminate bombing of civilian populations" and reiterated their commitment to a political transition in Syria, according to the US statement.

The United States has been launching airstrikes in Syria since 2014, targeting Islamic State and al-Qaeda-linked groups.

Russia started an aerial campaign in September to support al-Assad and has largely focused on rebel groups, including hardline factions but also moderate groups.

Moscow's air power has helped the government retake key pieces of territory. Thanks to its efforts, al-Assad's forces appear set to impose a siege on rebel-held eastern Aleppo city.

However, Islamic State dealt government forces a blow Tuesday when they took the key town of Khanasir, which straddles the government's only land route into the areas of Aleppo it controls.

The Syrian Observatory for Human Rights, a monitoring group, said the fall of the town came a day after the jihadists - who have lost ground to al-Assad's forces east of Aleppo - managed to cut off the road.

"The regime has been trying since yesterday to recapture the points that the Islamic State took over under Russian cover," said Rami Abdel Rahman, the head of the Observatory.

The Syrian war, which started in 2011 after al-Assad's forces launched a brutal crackdown against protests, has cost more than 250,000 lives and driven half the country's population from their homes, according to UN estimates.

The Syrian Observatory for Human Rights on Tuesday said it has now documented more than 270,000 deaths, including some 123,000 civilians, since mass protests broke out on March 18, 2011.

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