The United States and Russia were to hold new talks Wednesday on avoiding incidents in the skies of Syria, as regime forces launched heavy attacks against rebels near Damascus.
Fighting was also reported in the northern city of Aleppo, where jihadists from the Islamic State group were making advances against other rebel fighters.
The US-Russia talks come after the Pentagon said American and Russian planes had come within kilometres (miles) of each other on Saturday, making visual contact as the countries wage separate air wars over Syria.
Russia's air campaign, launched September 30, has raised fears of a military incident with the US-led coalition that has been bombing IS in Syria and Iraq for more than a year.
US Defence Secretary Ash Carter said the talks between the US and Russian militaries would aim to ensure Moscow follows "basic safety procedures" over Syria.
"Even as we continue to disagree on Syria policy, we should be able to at least agree on making sure our airmen are as safe as possible," Carter said.
Colonel Steve Warren, the Baghdad-based spokesman for the coalition, told reporters that Saturday's incident saw coalition and Russian planes just "miles apart" over Syria.
"Visual identification took place. All pilots conducted themselves appropriately and everyone went about their business," he said.
"But this is dangerous right?... There's always going to be some risk if there are uncoordinated actors in the battle space."
Despite the planned talks, Moscow said Wednesday that Washington had declined to host a high-ranking Russian delegation or to send a team to hold separate broader discussions on Syria.
"We have been told that they can't send a delegation to Moscow and they can't host a delegation in Washington either," Russian Foreign Minister Sergei Lavrov told parliament.
Russia's intervention has raised the stakes in the Syrian conflict, which has left more than 245,000 dead and forced millions from their homes since it erupted in March 2011.
Moscow insists it is targeting IS, which has emerged as the preeminent jihadist group during the conflict and seized swathes of territory in Syria and Iraq.
It said Wednesday that Russian jets had hit 40 IS targets in five Syrian provinces in the past 24 hours.
But Washington and its allies accuse Moscow of targeting moderate Western-backed rebels and seeking to prop up President Bashar al-Assad, a longtime Russian ally.
The regime began an operation Wednesday to dislodge insurgents entrenched on the outskirts of the capital, a military source told AFP.
"It began in Jobar with limited, precise and effective operations against lines of defence used by armed groups to observe the rest of the capital," the source said.
In Damascus on Tuesday, Russia's embassy was struck by two rockets reportedly fired from rebel-held territory on the eastern edges of the capital. There were no reports of dead or wounded.
Jobar neighbourhood, in eastern Damascus, has been a battleground for more than two years. Nearly all of its pre-war population has fled, and fighting between the Syrian army and rebel groups has devastated the suburb.
The Syrian army has attempted to retake it several times.
Residents of the area described heavy shelling and the Syrian Observatory for Human Rights, a Britain-based monitoring group, said warplanes had conducted at least eight raids on the area.
In Aleppo, the Observatory said IS had seized new territory from other rebels, blocking a key route between Aleppo and the Turkish border.
It said the fighting had killed 13 IS fighters and seven rebels.
"The rebels have suffered several reversals to IS in northern Aleppo and are caught between IS and the forces of the regime," said Maamun al-Khatib, editor of the rebel Shabha news agency in Aleppo.
Syrian regime forces have made key advances with the support of Russian air strikes.
Much of the latest fighting has focused on the strategic Sahl al-Ghab plain, a gateway to Assad's coastal heartland of Latakia.
Assad's forces are battling a wide range of opponents, including the jihadists of IS and Nusra, Western-backed rebel groups and Kurdish militia.
Washington has been increasing support for rebel groups it backs in Syria, but backing for Kurdish forces has rankled Turkey.
On Wednesday, Turkish Prime Minister Ahmet Davutoglu warned the United States and Russia against "unacceptable" military and political support for Syrian Kurdish forces fighting IS.
"We have a clear position. That position has been conveyed to the United States and the Russian Federation," Davutoglu said in televised comments.
"Turkey cannot accept any cooperation with terrorist organisations which have waged war against it."
Turkey considers the Democratic Unity Party (PYD), the main Kurdish group in Syria, an offshoot of the outlawed Kurdistan Workers' Party (PKK) which has waged a bloody insurgency against Ankara since 1984.