Beirut: The United States and Russia were to hold new talks on 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 launched air strikes over Syria on September 30, raising fears of accidental run-ins 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 in Boston after talks with his Australian counterpart.
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."
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.
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 launched heavy attacks including air raids today in a bid to dislodge insurgents entrenched on the outskirts of the capital, a military source told AFP.
"The army began a military operation this morning with the aim of expanding a security zone around areas controlled" by the government, the source said.