Syria’s army and main rebel force said they will cease fire today, in line with an inter­nationally backed truce during a Muslim holiday, but both reserved the right to respond to any aggression.

A peace initiative by UN and Arab League peace envoy Lakhdar Brahimi calls for a truce during the four-day Muslim holiday of Eid al-Adha from today marking the end of the hajj pilgrimage.

It was backed this week by the UN Security Council, and a spokesman for UN chief Ban Ki-moon said “the world is now watching” to make sure both sides stick by their commitment.

The army announced it would adhere to the ceasefire in a statement read on TV.

“On the occasion of Eid al-Adha, military operations will cease on Syrian territory as of Friday morning, until Monday the 29th,” it said.

But it also said the army would react if “armed terrorist groups continue to fire on civilians and government troops, attack public and private property and use car bombs and improvised explosive devices.”

It also warned of a response if rebels “strengthen their current positions or continue to receive reinforcements and ammunition” and to any fighters crossing from neighbouring countries.

The Free Syrian Army, chief among many rebel groups battling President Bashar al-Assad’s forces, responded positively soon afterwards, saying it too would lay down its weapons as long as regime troops adhere to the ceasefire.

“We will respect the ceasefire from tomorrow morning if the Syrian army does the same,” said General Mustafa al-Sheikh of the FSA, which had previously said it doubts Damascus would stand by any commitment.

“But if they fire a single shot, we will respond with 100. So we reserve the right to respond,” he said by telephone from Turkey.

He cautioned, however, that he could not speak on behalf of all rebel groups. “There is not a unified command for all the factions. We speak on behalf of a big enough number of fighters, but there are other armed factions who follow other commands,” Sheikh said.

An April ceasefire announced by Brahimi’s predecessor, former UN secretary general Kofi Annan, failed to take hold.If the latest one holds, it would mark the first real breakthrough in halting – even temporarily – the 19-month conflict that rights groups say has killed more than 35,000 people.

Eid al-Adha starts with prayers at dawn today, expected at around 5.30am local time in Syria.

Meanwhile shortly before yesterday’s announcements, there were no signs of a slowdown in the fighting, with rebels moving into a strategically important Kurdish neighbourhood in the main battleground city of Aleppo.

Residents in Aleppo’s Ashrafiyeh district said about 200 rebels had moved in to the area for the first time.

One said the rebels, who arrived on vehicles mounted with heavy machineguns and bearing the markings of the Liwa al-Tawhid main rebel unit, made it clear they were settling in for Eid despite the ceasefire promises.

“Snipers have set up in the buildings and 50 armed men, dressed in black and wearing headbands with Islamic slogans, entered a school near me. I heard them tell the residents: ‘We are here to spend Eid with you’,” he said.

“I am waiting for things to calm down before leaving,” he said.

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