Russian warships in the Mediterranean Sea fired cruise missiles at targets near Aleppo on Friday, days after Russia began to fly bombing missions from an airbase in Iran.
In northeastern Syria, warplanes from a US led coalition flew patrols on Thursday to protect local ground forces they back against Syrian regime air strikes that are targeting the Kurdish city of Hasaka, the Pentagon said.
Pentagon spokesman Captain Jeff Davis said, "the Syrian regime would be well advised not to interfere with coalition forces or our partners."
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 the Hamedan air base in Iran.
Russia's Defence Ministry said the strikes targeted the militant group Jabhat Fatah al-Sham, previously known as the Nusra Front until it broke formal ties with al Qaeda last month.
Plight of civilians "Nightmarish"
The World Food Programme has warned of a "nightmarish" situation after the plight of civilians in Aleppo was aggravated by dire shortages of basic goods in besieged areas.
Rebels and a war monitor said the regime helicopters had dropped incendiary barrel bombs in Daraya, a suburb of Damascus, early on Friday, putting the opposition-held town's only hospital out of action.
— Brett (@Gen_Juhziz) August 18, 2016
Assad's most powerful military ally Russia has 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 UN monitored the truce and enforced compliance.
Dozens killed in Hasaka
On Friday, thousands of civilians have been evacuated from areas of Hasaka following regime air strikes, YPG, which is the Syrian affiliate of the PKK terrorist group, said.
— Mutlu Civiroglu (@mutludc) August 18, 2016
Hasaka is divided into zones of YPG and Syrian regime control. Fighting between the zones has killed dozens of civilians in the past 48 hours, according to the local sources.
Pentagon official said on Friday a US led coalition sent aircraft into northeastern Syria on Thursday in a "very unusual" move in order to protect American special operation ground forces from attacks by regime jets.
US scrambled jets over N. Syria after 2 Syria Su-24s bomb dangerously close to US SOF on the ground. Say Dod Spox Capt. Davis.
— Jamie McIntyre (@jamiejmcintyre) August 19, 2016
Captain Jeff Davis, the Pentagon spokesman told reporters the coalition aircraft reached the area around the city of Hasaka as the two regime SU-24s were leaving, and the US special operation forces were in the area where the strikes were taking place. Regime planes did not respond to efforts by ground forces to contact them, he added.
The spokesman denied he was aware of any other instances where coalition aircraft had been scrambled to respond to Syrian regime bombing.
On Friday, two regime aircraft tried to pass through the airspace around Hasaka, but left without incident when they were met by coalition fighter jets. The coalition fighter jets were F-22 aircraft and came within 1 mile (1.6 km) of the regime planes.
Rebel groups, including Jabhat Fatah al-Sham, stormed a regime army complex in southwest Aleppo two weeks ago, breaking a siege on opposition-held parts of Aleppo and prompting fierce counter-attacks.
There was a "positive atmosphere" surrounding talk of a ceasefire. "But so far there are no details," a senior rebel commander said.
A military source said in the last 24 hours regime warplanes had carried out 46 sorties, including strikes in Aleppo that destroyed a tank, a vehicle loaded with ammunition and three mortar emplacements, and killed dozens of rebel fighters.
Continuing clashes between rebels and the regime army backed by foreign militias were fiercest in the southwest of city, the Syrian Observatory for Human Rights, a UK 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 UN words, but that's what it is."