'Dissent channel' cable with 51 signatures questions US policy on Syria, and comes as Russia is accused of multiple ceasefire violations
Dozens of US diplomats have said their country should launch military strikes against the Syrian government, breaking ranks with President Barack Obama's policy on the war.
The so-called "dissent channel" cable urges attacks against Bashar al-Assad's regime for its persistent violations of a shaky ceasefire aimed at bringing an end to the five-year conflict.
The cable, signed by 51 working-level diplomats, calls for "a judicious use of stand-off and air weapons", according to the New York Times, and lays bare the divisions in Washington policy circles.
Russia on Friday said the thrust of the message was "obviously at odds with UN resolutions" on Syria.
The State Department cable says Assad's continued violation of a countrywide partial ceasefire - a cessation of hostilities - that was negotiated in February, meant a political settlement was untenable, the Times reported.
Without consequences for the violations, the Syrian government will feel no obligation to talk with moderate opponents, the cable said, with their barrel bombing of civilians the "root cause of the instability that continues to grip Syria and the broader region."
"We are aware of a dissent channel cable written by a group of State Department employees regarding the situation in Syria," State Department spokesman John Kirby told AFP.
"We are reviewing the cable now, which came up very recently," he added.
Obama's preferred policy - to work with Russia to secure a ceasefire in Syria's five-year civil war and talks on a political transition - received a further blow when Assad's Russian allies launched raids in southern Syria, according to the Pentagon.
"Today, Russian aircraft conducted a series of airstrikes near al-Tanf against Syrian Counter-ISIL forces that included individuals who have received US support," said a senior US defence official who requested anonymity.
"Russia's latest actions raise serious concern about Russian intentions. We will seek an explanation from Russia on why it took this action and assurances this will not happen again."
More barrel bombs fell on rebel-held areas of Aleppo on Thursday, according to activists, in contravention of a ceasefire called by Russia on Wednesday.
At least four civilians were killed - three on the eastern side of Aleppo and one in the west, the Syrian Observatory for Human Rights said.
Barrel bombs 'root cause of instability'
US policy on Syria has been criticised for ineffectiveness, after five years of brutal internecine fighting that has claimed hundreds of thousands of lives and sent many more fleeing.
US Secretary of State John Kerry did hint this week that Washington could be on track to pursuing a more robust policy on Syria, telling a conflict resolution conference in Norway that he was running out of patience with Russia.
"Russia needs to understand that our patience is not infinite," Kerry said on Wednesday at the Oslo Forum. "In fact, it is very limited now with respect to whether or not Assad is going to be held accountable.
"The United States is not going to sit there and be used as an instrument that permits a so-called ceasefire to be in place while one principal party is trying to take advantage of it to the detriment of the entire process. We're not going to allow that to continue."
American forces do not confront Assad’s Russian and Iranian-backed troops in Syria, but are engaged in assisting local militias to fight Islamic State.
The US is also leading a coalition that has been bombing Islamic State positions since September 2014.
However, CIA chief John Brennan on Thursday admitted that, "despite all our progress against [IS] on the battlefield and in the financial realm, our efforts have not reduced the group's terrorism capacity and global reach".