Air strikes on rebel-held areas of Syria's second city Aleppo and a town to its west killed at least 19 people on Tuesday, emergency workers said.

The strikes came after rebel shelling killed at least 19 civilians in government-held districts of Aleppo on Monday and are the latest in a surge of violence in and around the city that has severely tested a February 27 ceasefire.

At least six people, half of them children, were killed in regime and rebel bombardment of Syria's northern city of Aleppo on Monday, civil defense officials and a monitor said.

Three civilians, including a child, were killed when the regime fired two rockets on the rebel-controlled east of the city, the civil defense said.

Syrian President Bashar Assad met with an Algerian minister on Monday, in a rare Arab ministerial visit to the war-torn country, state media reported.

Assad thanked Algeria's minister for the Arab League, Abdelkader Messahel, that Algiers was standing by Damascus in the five-year conflict, SANA news agency reported.

At least 27 civilians were killed Saturday in regime bombardment on rebel-held areas across Syria, a monitor and local sources said, in the latest deadly violence despite a ceasefire deal.

Twelve civilians were killed in Aleppo, according to a local civil defense official, and the Syrian Observatory for Human Rights said 13 others died in shelling on the rebel town of Douma, east of Damascus.

Syria's fragile ceasefire is in grave peril, U.N. President Barack Obama and the U.N.'s special envoy warned Friday, as violence surged in the war-ravaged country's second city Aleppo.

The truce "is still in effect, but it is in great trouble if we don't act quickly", the United Nations' top envoy for Syria, Staffan de Mistura, told reporters in Geneva, where he is mediating faltering peace talks.

The Islamic State jihadist group on Friday captured a Syrian pilot alive after shooting down his plane east of Damascus, the IS-affiliated Amaq news agency said.

Amaq gave the pilot's name as Azzam Eid, from Hama. It said IS fighters had shot down his plane and found him alive after he parachuted down to the crash site.

Fifty Syrian pro-government fighters surrendered Thursday to Kurdish forces in the northeastern city of Qamishli, a Kurdish security source said as fighting raged for the second consecutive day.

"A group of fighters loyal to the regime were taking cover in a prison in Qamishli, and Kurdish forces gave them until noon to hand themselves in," a Kurdish security source told AFP.

The family of U.S. journalist Steven Sotloff, who was beheaded by the Islamic State group, has sued Syria on grounds it provided material support to the jihadist organization.

In a suit filed Monday in U.S. District Court in Washington, Sotloff's parents and sister seek $90 million in damages from the government of President Bashar Assad.

Twenty-five men were evacuated from a rebel-held stronghold west of the Syrian capital Wednesday, an AFP reporter said, as part of a U.N.-backed evacuation of 500 people from four besieged towns.

The AFP reporter said 15 of the men evacuated from Zabadani were anti-government rebels and another 10 were elderly in need of medical attention.

The Islamic State group has tightened the noose on a regime-held enclave in eastern Syria, overrunning part of the city of Deir Ezzor and advancing on its vital airbase, a monitor said Wednesday.

"IS seized the Al-Sinaa neighborhood of Deir Ezzor on Tuesday evening and fighting is continuing on the edge of the airport," the Syrian Observatory for Human Rights said.