At least 55 people have been killed over the past 24 hours in the Syrian province of Aleppo as Russian air radis resumed over the countryside, sources told Al Jazeera.

The deaths from raids in Aleppo's Bustan al-Qasr and Fardos neighbourhoods, as well as Ar-Rasheeqa city, were the results of the heaviest Russian bombardment in days on the city's rebel-held sector, people in the city said. Dozens were also wounded.

The Syrian Observatory for Human Rights, a UK-based group that monitors the war through a network of informants in the country, reported a lull in fighting on Wednesday. 

"Silence is taking over Aleppo city, so far no gunshots or airstrikes have been heard, since the airstrike[s]," the monitor said.

Aleppo onslaught: Heaviest Russian raids in days

The latest bout of violence came as diplomatic efforts to stop the war intensified.

Al Jazeera has seen a letter signed by at least 62 countries, including Saudi Arabia and Qatar, calling on the UN Security Council to prevent further deaths from a "calculated campaign".

The letter warns that the war in Syria is unlikely to be resolved by armies. It called on all parties to adopt political processes for a political transition based on recommendations agreed upon at Geneva conventions and by the Security Council. 

"The fighting, the bombardment and the siege have worsened conditions inside Aleppo," said Al Jazeera's Zeina Khodr, reporting from the Turkish border with Syria.

"The United Nations is warning that drinking water remains limited, medical supplies are urgently needed and the distribution food rations are being split in half ... aid agencies are appealing for access to treat the wounded," Khodr added.

READ MORE: Letter from Aleppo - 'My city is not just a death toll'

Meanwhile, Turkey's military said that fighters from the Islamic State of Iraq and the Levant (ISIL, also known as ISIS) group were putting up "stiff resistance" in Syria's north to attacks by Turkish-backed rebel fighters.

Turkey launched an incursion two months ago to drive ISIL fighters away from its border.

Supported by Turkish tanks and air strikes, the rebels have been pushing towards the ISIL stronghold of Dabiq. 

"Due to stiff resistance of the [ISIL] terror group, progress could not be achieved in an attack launched to take four settlements," Turkey's military said, naming the areas east of the town of Azaz as Kafrah, Suran, Ihtimalat and Duvaybik.

The operation to drive the fighters away from the Turkish border, dubbed "Euphrates Shield", was in its 50th day on Wednesday. The Turkish-backed rebels have seized control of about 1,100 square km (425 square miles) of territory from ISIL since the operation began, the military said.

In a daily round-up, it said 19 Islamic State fighters had been "neutralised" in clashes, while eight Turkish-backed rebels were also killed. Twenty-two rebels were wounded but Turkish forces did not suffer any casualties.

Turkish fighter jets destroyed five buildings used by ISIL, while US-led coalition jets "neutralised" 28 of the fighters and destroyed three buildings, it said.

The operation has also targeted a Kurdish group whose presence along its border Turkey sees as a threat.

Hundreds of thousands of Syrians have died the war, which has extended five years, and has displaced millions.

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Source: Al Jazeera News And Agencies