The US and Russia have agreed to extend a fragile ceasefire to Aleppo, one of the worst-affected provinces in Syria. The agreement, which was reached on 3 May, has already come into effect.

The truce comes into force even as the UN accused President Bashar al-Assad's regime of carrying out war crimes. The US State Department said the parties involved have consented to cease all hostilities from 4 May midnight but the Syrian regime said the truce would begin on 5 May at 1am. Details of the agreement remain murky as there has been no word on the duration of the truce and whether there are any conditions attached to it.

US Secretary of State John Kerry said in a statement: "We expect all the parties to the cessation of hostilities to fully abide by the renewed cessation in Aleppo. That means the regime and the opposition alike."

"We will continue to work to de-escalate the violence all across Syria in hopes of being able to move back to the talks as soon as people have a confidence that this is really taking hold, and that we have laid the groundwork for the ability to be able to go back," he added. Sporadic fighting was reported in Aleppo until the final moments of the ceasefire arrangement.

Whether the truce, which was clinched after days of diplomatic wrangling, proves to be successful depends on how durable the agreement is. The Aleppo deal was reached after reaffirmations from the US and Russia that though the ceasefire remained volatile in several parts of the country, they would try to extend it.

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However, the cessation of hostilities does not apply to Islamic State (Isis) and al-Qaeda-affiliated Nusra Front as they have never been part of the negotiations. Assad's Syrian government said the "regime of calm" would be implemented in Aleppo. Russia's defence ministry spokesperson Igor Konashenkov has also released a formal statement confirming the agreement.

Meanwhile, top UN officials have yet again accused Assad of carrying out war crimes in the strife-torn country. UN's humanitarian aid chief Stephen O'Brien and political affairs official Jeffrey Feltman told the UN Security Council that Assad should face a trial in The Hague. "Those responsible for war crimes must be held to account," said Feltman.