More than 50 US diplomats have signed an internal document critical of the government's policy in Syria and calling for military strikes against President Bashar al-Assad's government.
The "dissent channel cable" was signed by 51 mid to high-level State Department officials involved in advising on Syria policy.
The document calls for "targeted military air strikes" against Assad's government, as reported by the Wall Street Journal, which broke the story first on Thursday, citing copies of the cable.
US President Barack Obama has thus far authorised strikes against the Islamic State of Iraq and the Levant (ISIL) and other US-designated armed groups in Syria.
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Military strikes against the Syrian government, however, would mean a major shift in the Obama administration's reluctance to intervene directly in catalysing regime change.
The US has stationed about 300 special operations forces on the ground in Syria, training "moderate" Syrian rebel groups and targeting ISIL but not the Assad government.
Officials said the cable is unlikely to alter Obama's current policy on Syria, as the White House remains opposed to deeper US military involvement in the conflict, Reuters news agency reported.
The US diplomats' document was revealed on the same day that John Brennan, the CIA director, told a congressional hearing that Assad is in a stronger position than he was a year ago, thanks to Russian air strikes.
Brennan said on Thursday that ISIL, also known as ISIS, remains "formidable" and "resilient" despite the US-led international coalition's efforts to defeat it militarily, adding that the group has tens of thousands of fighters around the world - far more than al-Qaeda had at its height.
"Unfortunately, despite all our progress against ISIL on the battlefield and in the financial realm, our efforts have not reduced the group's terrorism capability and global reach," he said.
"In fact, as the pressure mounts on ISIL, we judge that it will intensify its global terror campaign to maintain its dominance of the global terrorism agenda."
Meanwhile, on the ground fighting continues unabated in several parts of Syria.
Government air strikes
At least seven people have been killed in government air strikes on rebel-held areas in Syria's northern city of Aleppo in recent days, according to a monitoring group.
About 70 fighters were also reportedly killed in battles around the city, the group said.
The attacks came just hours after Russia declared a two-day ceasefire in Aleppo.
Elsewhere, Russian fighter jets continued to strike Syrian rebels - some backed by the US - near al-Tanf, in southern Syria.
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Al Jazeera's Kimberly Halkett, reporting from Washington DC, said these air strikes "raise serious concern in Washington about Russia's intentions".
"We will seek an explanation from Russia on why it took this action and assurances this will not happen again," a US official told Reuters.
The US has consistently refused to join forces with Russia in Syria against ISIL ever since Russia launched its campaign of air strikes in September 2015, accusing it of acting solely to prop up Assad.
The US has called on Assad to step down.
Source: Al Jazeera and agencies