A "catastrophic deterioration" of the situation in Syria is threatening the delivery of aid to millions of people, the U.N. warned Thursday, urging immediate international action to shore up humanitarian access on the ground.

"The stakes are so incredibly high," said Jan Egeland, who heads an international humanitarian taskforce for the war-ravaged country.

An aid convoy for besieged civilians entered a rebel-held town in central Syria on Wednesday for the first time in a year, the Red Cross said.

In the third such aid operation in Homs province this week, the Red Crescent and International Committee of the Red Cross was delivering food parcels to 12,000 families in and around Talbisseh, ICRC spokesman Pawel Krzysiek said.

Syrian Kurdish forces have retrieved the remains of a Russian special forces officer from the Islamic State jihadist group, a source on the ground told AFP on Wednesday.

"The Kurdish People's Protection Units (YPG) managed to retrieve the body of a Russian soldier killed during the battle for Palmyra that was in the hands of IS," the source said, without saying how.

Relief groups on Monday began their second major aid delivery in a week to tens of thousands of besieged people in central Syria, a Red Cross spokesman told AFP.

Thirty-five aid trucks would be delivered to Rastan and surrounding rural areas in central Homs province, said International Committee of the Red Cross spokesman Pawel Krzysiek.

A car bomb killed at least seven people near the revered Shiite shrine of Sayyida Zeinab, south of the Syrian capital Damascus, on Monday, state television reported.

The bomb struck Al-Diyabiyah, a town that serves as one of the gateways for the many pilgrims from around the world who visit the Shiite holy site.

Syrian government representatives flew in from Damascus on Friday to try to calm three days of deadly clashes with Kurdish militia in the northeastern city of Qamishli, a security official said.

The fighting in the mainly Kurdish city, most of which is held by the militia, has been a distraction from the two sides' common struggle against the Islamic State jihadist group, which controls swathes of territory to the south.

U.N.-brokered talks aimed at ending Syria's brutal conflict will push ahead until April 27, despite the departure of the main opposition group, a negotiator in the talks said Friday.

Syria's main opposition High Negotiations Committee (HNC) earlier this week halted its formal participation in the round of talks that began in Geneva on April 13 in frustration over surging violence on the ground.

Air strikes on rebel-held neighborhoods in Syria's second city Aleppo on Friday killed at least 18 civilians and wounded more than a dozen, the local civil defense told AFP.

The Syrian Observatory for Human Rights said regime warplanes carried out the air strikes and gave a toll of 19 dead, up from its earlier toll of 10.

Aid groups launched their largest aid delivery yet in war-torn Syria on Thursday after the U.N. evacuated hundreds of besieged residents, intensifying relief efforts even as peace talks falter.

The two major operations were a rare sign of humanitarian progress in Syria, where the brutal five-year war has left 270,000 dead as a partial ceasefire hangs by a thread.

More rockets hit a Turkish town on the Syrian border on Tuesday, wounding at least three people, a day after five were killed by fire from an area of Syria controlled by jihadists, a Turkish government official said.

In the last few weeks, Islamic State (IS) jihadists have repeatedly fired rockets at Kilis, the only town in Turkey where refugees from Syria's five-year conflict now outnumber local Turks.