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 rebel fighters.
The planned talks come after US and Russian planes passed within kilometres (miles) of each other on Saturday.
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.
Colonel Steve Warren, the spokesman for the US-led coalition, told reporters that coalition and Russian planes came 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."
Moscow confirmed the incident, saying one of its fighters had approached a coalition plane after detecting "emissions from an unidentified flying object."
"Our fighter turned and flew to a distance of two to three kilometres (one to two miles from the plane), not with the aim of scaring someone, but to identify the object in question and to whom it belonged," defence ministry spokesman Igor Konashenkov said.
- Offensive near Damascus -
Despite the planned talks, Moscow said Washington had declined to host a high-ranking Russian delegation or to send its own 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 tensions with Washington, and Germany cautioned Wednesday against a full-blown conflict between the two countries.
German Foreign Minister Frank-Walter Steinmeier, who is to visit Iran, Saudi Arabia and Jordan in the coming days, said he wanted to "urgently caution the US and Russia not to militarily engage in a way that in the end could lead to a conflict".
Russia says its aerial campaign is targeting IS, and on Wednesday said its jets had hit 40 targets belonging to the jihadist group in five Syrian provinces in the past day.
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But Washington and its allies accuse Moscow of targeting moderate Western-backed rebels and propping up President Bashar al-Assad, a longtime Russian ally.
Syria's regime has managed to advance in the strategic Sahl al-Ghab region that is a gateway to Assad's coastal heartland of Latakia thanks to Russian air support.
And on Wednesday regime forces began an operation to dislodge insurgents from the Jobar area on the eastern outskirts of Damascus, a military source told AFP.
A day earlier, Russia's embassy was hit by two rockets reportedly fired from rebel-held territory on the eastern edges of Damascus. There were no reports of dead or wounded.
Jobar has been a battleground for more than two years, with the army trying repeatedly to retake it from rebels.
It has been devastated by fighting and most of its residents have fled.
Eyewitnesses described heavy shelling on the area in the morning and the Syrian Observatory for Human Rights, a Britain-based monitoring group, said warplanes had conducted at least eight raids.
- Turkey warns on Kurds -
In northern Syria, meanwhile, the Observatory said IS had seized territory in Aleppo province from rebels, blocking a key route between Aleppo city 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.
Syria's war began in March 2011 with anti-government protests but has since evolved into a complex multi-front battle involving the regime, rebels, jihadists and Kurdish forces.
The Kurds have emerged as a key force fighting IS and a leading partner for the US-led coalition, but their rising profile 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.
"Turkey cannot accept any cooperation with terrorist organisations which have waged war against it," he said.
Turkey considers the main Kurdish group in Syria to be an offshoot of the outlawed Kurdistan Workers' Party (PKK) which has waged a bloody insurgency against Ankara since 1984.