At least 14 civilians have been killed and several others wounded in a series of barrel-bomb attacks on opposition-held areas near Damascus, Al Jazeera has learnt.

Syrian regime air strikes targeted the town of Wadi Barada, northwest of the capital, on Monday, injuring dozens of people, including several children, activists told Al Jazeera.

According to videos posted on social media by opposition groups, the Syrian regime intensified its assault on the town.

The White Helmets, an all-volunteer rescue group, said a number of people were trapped under the rubble following the heavy bombardment.

Barrel bombs are crude, unguided explosive devices used by Syrian regime forces that have been blamed for thousands of civilian deaths.

They are typically constructed from large oil drums and filled with explosives, nails and scrap metal.

Third day of offensive

Al Jazeera's Hashem Ahelbarra, reporting from Gaziantep, on the Turkish side of the border with Syria, said the regime was in the third day of an offensive against Jaish al-Islam, which controls Wadi Barada and is one of the most powerful groups battling Bashar al-Assad's forces.

On Friday, the Syrian army and the Lebanese Shia armed group Hezbollah launched an operation to retake Wadi Barada, after accusing the rebels of contaminating drinking water at a spring with diesel.

The Wadi Barada valley, a mountainous area near the Lebanese border, has been under siege since 2014 with food, water and electricity all in short supply.

In a separate development, Turkey has urged the US to step up its air strikes against ISIL group operating in the town of al-Bab in northern Syria on the border with Turkey.

"Turkey has been facing some fierce resistance around the town of al-Bab, which is the largest ISIL-held town north of Aleppo," Al Jazeera's Charles Stratford, reporting from Gazientep, said.

"The Turkish government is now calling for more effort from US coalition air strikes on that border area."

The battle to retake al-Bab is part of a wider Turkish effort to clear its border area with Syria from ISIL control.

Over 215 residential areas, including Jarablus city in northern Syria, have been cleared of ISIL fighters, according to the Turkish military.

Displacement figures

The Syrian regime took back previously opposition-held eastern Aleppo last week after an intense battle that led to the displacement of tens of thousands of civilians.

Many were also caught in the crossfire.

According to a statement from the Turkish Disaster and Emergency Management Authority (AFAD), 250 wounded Syrians hailing from Aleppo were transferred to hospitals in Turkey.

Only those severely injured were allowed to cross, while no civilians have passed the border, the statement read.

Of the 250, 35 have died while hospitalized, and 75 remain in critical condition.

Forced disappearances

The Syrian war started as a largely unarmed uprising against Assad in March 2011, but quickly developed into a full-on armed conflict.

Calculating a precise death toll is difficult, partially owing to the forced disappearances of tens of thousands of Syrians whose fates remain unknown.

Almost 11 million Syrians - half the country's pre-war population - have been displaced from their homes.

According to the latest report published by the Netherlands-based PAX aid group and the Washington-based Syria Institute, more than one million Syrians are believed to be trapped in at least 39 communities across the country.

However, the figures are likely to keep changing as Assad's forces gain more ground.

PHOTO CAPTION

A general view shows damage in the Old City of Aleppo, Syria December 13, 2016. REUTERS

Al-Jazeera