US President Barack Obama said on Friday in London that he was "deeply concerned about the cessation of hostilities fraying and whether it's sustainable."

A landmark partial ceasefire, which was negotiated by the United States and Russia and took effect in Syria on February 27, had dramatically curtailed violence across much of Syria and raised hopes that a lasting deal could be struck in Geneva to end the five years of bloodshed.

The country has been rocked by fighting in recent weeks, however, particularly around the northwestern city of Aleppo. On Friday alone, at least 25 civilians were killed and another 40 wounded in airstrikes on rebel-held neighborhoods.

The United Nations envoy for Syria, Staffan de Mistura, also expressed his concerns to reporters in Geneva on Friday, saying that the truce in Syria was "still in effect, but it is in great trouble if we don't act quickly."

'Very, very productive' meetings

Frustrated by the increasing violence, the lack of access for desperately needed aid and the failure to secure the release of detainees, Syria's main opposition High Negotiations Committee (HNC) withdrew from peace talks in Geneva earlier this week.

De Mistura said on Friday, however, that members of his team had continued to hold "very, very productive" meetings at a technical level with remaining HNC members at their Geneva hotel. The most recent round of talks, which began on April 13, is due to continue until Wednesday.

A major sticking point in the indirect talks is the fate of Syrian President Bashar al-Assad. While the opposition has demanded that any peace deal include his departure, Damascus insists that his future is non-negotiable.

IS allegedly captures pilot

The concerned comments on Friday came as the "Islamic State" (IS) jihadi group claimed to have captured a Syrian pilot after shooting down his plane. IS allegedly captured the pilot, identified as Azzam Eid from Hama, after he parachuted to the crash site.

Several Syrian government warplanes have been targeted by IS fighters in recent weeks. In two cases over Dmeir military airport near Damascus, and in the southern province of Sweida, both pilots were able to land in regime-held zones.

In December 2014, IS shot down a warplane belonging to the US-led coalition and captured Jordanian pilot Maaz al-Kassasbeh. The jihadi militants later posted video footage of the man being burned alive.

Civilians killed in US strikes

As the fight against IS continues, the Pentagon acknowledged on Friday that 20 civilians had been killed in US airstrikes against the militant group between September 10, 2015, and February 2, 2016.

Observers rejected the figures, however, deeming them "unbelievable," given the intensity of the 20-month-old air campaign, which has seen some 12,000 plane and drone strikes carried out over Syria, often in urban areas.

ksb/sms (Reuters, AFP, AP)