Russian warships in the Mediterranean Sea fired cruise missiles at targets near Aleppo on Friday, a further sign of Moscow's broadening military effort in Syria days after it began to fly bombing missions from an airbase in Iran.
Russian air power had helped Syrian President Bashar al-Assad make steady advances against rebels seeking to oust him since Moscow's intervention a year ago, but a recent insurgent advance in Aleppo has checked that momentum.
In northeastern Syria, warplanes from a U.S.-led coalition flew patrols on Thursday to protect local ground forces they back against Syrian government airstrikes that are targeting the Kurdish city of Hasaka, the Pentagon said.
"The Syrian regime would be well advised not to interfere with coalition forces or our partners," Pentagon spokesman Captain Jeff Davis said.
Russia's three cruise missile launches were its first against targets in Syria from the Mediterranean, with previous ones made from its Caspian Sea fleet. On Tuesday Russian bombers began flying missions in Syria from Hamedan air base in Iran.
Russia's Defense Ministry said the strikes targeted the Islamist militant group Jabhat Fatah al-Sham, known as the Nusra Front until it broke formal ties with al Qaeda last month before playing a big role in the sudden rebel advances in Aleppo.
The upsurge in fighting and airstrikes in and around the city, split between government-held west and rebel-held eastern sectors, has prompted growing international concern, galvanized by pictures on Thursday of a dazed, bloodied child.
The plight of civilians in Aleppo has been aggravated in besieged areas by dire shortages of basic goods, leading the World Food Programme to warn of a "nightmarish" situation.
In Daraya, a suburb of Damascus, rebels and a war monitor said the Syrian army's helicopters had dropped incendiary barrel bombs early on Friday, putting the opposition-held town's only hospital out of action.
On Thursday Russia, Assad's most powerful military ally, said it supported a proposal for a weekly 48-hour pause in fighting in Aleppo to allow aid to reach the besieged areas and that it was ready to start the first one next week.
On Friday, the main umbrella group for the Syrian opposition also cautiously welcomed the idea provided the U.N. monitored the truce and enforced compliance. During a previous humanitarian pause this year, both sides complained the other had broken the truce as fighting escalated again.
Dozens killed in Hasaka
On Friday Syrian Kurdish authorities evacuated thousands of civilians from Kurdish areas of Hasaka following government air strikes, a spokesman for the Kurdish YPG militia, an integral part of the U.S.-backed Syrian Democratic Forces (SDF), said.
A civilian removes the rubble in front of a damaged shop after an airstrike in the rebel held al-Saleheen neighborhood of Aleppo, Syria August 18, 2016.
The SDF is at the heart of Washington's military campaign against Islamic State group and last week seized the town of Manbij from the militant organization, part of a growing swathe of northern Syria it controls.
The U.S.-led coalition aircraft arrived at Hasaka as two Syrian SU-24s, which had carried out the strikes, were leaving. "This is very unusual, we have not seen the regime take this kind of action against YPG before," the Pentagon's Davis said.
Hasaka is divided into zones of Kurdish and Syrian government control and fighting between them has killed dozens of civilians in the past 48 hours, YPG spokesman Redur Xelil said.
The YPG and the government have mostly avoided confrontation during the multi-sided war that has turned Syria into a patchwork of areas held by the state and an array of armed factions.
The Syrian army said on Friday that this week's fighting was caused by Kurdish security forces attempting to take over Hasaka, prompting it to return fire on armed groups.
Assad, backed by Russia and Iran, has focused mostly on fighting Sunni Arab rebels who have been battling to oust him in western Syria with support from countries including Turkey, Saudi Arabia and the United States.
The YPG, or People's Protection Units, has meanwhile prioritized carving out and safeguarding predominantly Kurdish regions of northern Syria. The group has ties to Kurdistan Workers Party (PKK) rebels in Turkey.
While the YPG controls most of the northeast, the Syrian government has maintained footholds in the cities of Hasaka and Qamishli at the border with Turkey.
The SDF seizure of Manbij from Islamic State last week raised the prospect of possible advances towards al-Bab, near Aleppo.
Families of Islamic State fighters in al-Bab and another nearby town, Jarablus, were evacuated to the militant group's stronghold of Raqqa, the Syrian Observatory for Human Rights, a U.K.-based war monitoring group, said.
Rebel groups, including Jabhat Fateh al-Sham, stormed a Syrian army complex in southwest Aleppo two weeks ago, breaking a siege on opposition-held parts of Aleppo and prompting fierce counter-attacks.
A still image, taken from video footage and released by Russia's Defence Ministry on August 19, 2016, shows a rocket being launched from a Russian corvette in the Mediterranean Sea at Jabhat Fateh al-Sham militant targets in Syria, according to the ministry. Ministry of Defence of the Russian Federation/Handout via Reuters.
A senior rebel commander said there was a "positive atmosphere" surrounding talk of a ceasefire. "But so far there are no details."
Syrian warplanes had carried out 46 sorties in the last 24 hours, including strikes in Aleppo that destroyed a tank, a vehicle loaded with ammunition and three mortar emplacements, and killed dozens of rebel fighters, a military source said.
Continuing clashes between rebels and the Syrian army and allied militias were fiercest in the southwest of city, the Syrian Observatory for Human Rights, a U.K.-based monitor of the five-year-old civil war, said on Friday.
It added that air strikes and shelling in and around Aleppo had killed 422 civilians, including 142 children, this month.
"We need a 48-hour pause, we need it now," WFP spokeswoman Bettina Luescher told a briefing in Geneva on Friday. While the rebel advance this month opened a narrow corridor into opposition-held areas of Aleppo, access remains very limited and dangerous, meaning aid supplies are scarce.
"It's crucially important that we go in there because people are absolutely desperate," Luescher added. "From both sides, these sieges have to stop - it's inhumane, awful, disgusting, nightmarish. Not necessarily U.N. words, but that's what it is."