John Kerry, the US secretary of state, has warned the Syrian regime and its main military backer Russia to respect "frayed" ceasefire and said that its "patience was not infinite" amid mounting death toll.

"Russia needs to understand that our patience is not infinite. In fact it is very limited with whether or not [Bashar] al-Assad is going to be held accountable," Kerry said on Wednesday after meeting with his Iranian counterpart Mohammad Javad Zarif in the Norwegian capital, Oslo.

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Kerry also met defence ministers from Syria, Iran and Russia in an effort to halt the fighting, particularly in the northern city of Aleppo, where raging battle has claimed dozens of lives in the past two days.

"We also are prepared to hold accountable members of the opposition" who have been involved in continuing violence, he said.

The Syrian conflict has drawn in world powers, with the US, along with regional powers, largely backing the moderate rebels while Russia began a military offensive in support of the Assad regime in September.

A Moscow and Washington-backed ceasefire has been in place since February 27, but a fresh bout of fighting broke out in April that stalled the UN peace talks in Geneva.

More than 280,000 people have been killed since the start of Syria’s war in 2011, and millions have been displaced, the UN estimates.

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"It is very clear that the cessation of hostilities is frayed and at risk and that it is critical for a genuine cessation to be put in place," Kerry said. "We know that, we have no illusion."

Efforts are under way, he said, to reach a new agreement "in the next week or two" to reinstate the ceasefire across Syria, leading to more humanitarian aid deliveries and a resumption of the peace process.

The Assad’s regime has stepped up its military campaign against the ISIL, also known as ISIS, and rebels, especially in the city of Aleppo.

Russian forces have also attacked many rebel groups and civilian areas under the justification of attacking ISIL and al-Nusra Front - both excluded from the ceasefire.

Washington has been pushing for cessation of hostilities amid deteriorating humanitarian situation.

"This is a critical moment and we are working very, very hard to see if we can in the next week or two come to an agreement that has a capacity to more fully implement a ceasefire across the country and deliver humanitarian access in a way that then provides for a genuine opportunity to bring people to the table and start talking about a transition," he said.

"I'm not going to make any promises to be delivered on but I do believe the conversation I had with Zarif indicates to me possibilities for how this could be achieved," Kerry said.

Earlier this month, the UN said the Syrian government had granted preliminary aid access to 15 of 18 besieged areas, where nearly 600,000 people live surrounded mostly by government forces.