Washington (AFP) - With diplomacy faltering and Aleppo under siege, President Barack Obama is considering fresh Syria sanctions that could claw deeper into the regime and target its Russian backers.

Officials and diplomats said the strategy is still being thrashed out, but initial efforts could focus on passing UN sanctions against those implicated in chemical weapons attacks.

A UN-backed panel is expected in the next few weeks to present new findings about deadly chemical attacks in 2014 and 2015.

The panel -- formed by the United Nations and the independent Organization for the Prohibition of Chemical Weapons -- has already pinned the blame on the Syrian Air Force.

But the latest report, due before October 27, is expected to go into more detail about who is responsible, paving the way for targeted sanctions.

Supporters say the sanctions would send a signal that despite years of fighting, innumerable atrocities and at least 300,000 deaths, some small measure of accountability in Syria remains.

And while most of President Bashar al-Assad's inner circle and top military aides are already the subjects of US travel bans and assets freezes, officials believe that targeting lower-level military officers would have a chilling effect on Syrian military morale.

But the greatest impact may be diplomatic.

A demand for sanctions would put Russia in the uncomfortable position of defending chemical weapons use by its ally, and could force Moscow to wield its veto in the UN Security Council.

The report had earlier been delayed, partly to provide space for US and Russian efforts to broker a ceasefire.

Those have now failed, heaping pressure on Obama to act to stop the carnage in Syria.

"What we are doing now is a different type of diplomacy -- one which might be more robust -- it could be resolutions which are designed to put pressure on them," a Security Council diplomat said.

"The strategy that we are on is to try to change Russian behavior and let's face it we have not been very successful at that in the year that they have been militarily supporting Assad," the diplomat added.

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"There is already work going on a draft resolution to take forward the 3rd and further 4th report in terms of what we do about it."

- Gaining leverage -

If the UN route fails, attention is likely to turn to sanctions agreed by the United States, the European Union and other allies.

Officials indicated the scope could be broad, covering not only Syrians but also Russian firms that provided the means to carry out the bombing of civilian areas.

That would be intended as a strong signal to Moscow that it is not immune and continued support for the regime would come at a price.

"I wouldn't rule out multilateral efforts outside of the UN to impose costs on Syria or Russia or others with regard to the situation inside of Syria," said White House spokesman Josh Earnest.

"I wouldn't take that off the table in terms of options that the president may consider."

The sanctions would be targeted at specific firms -- aircraft parts suppliers or chemicals producers -- to overcome opposition in European capitals to broad sanctions against Russia, a major trading partner.

But with Russia and Syria dropping bombs on besieged Aleppo and a humanitarian crisis of historic proportions looming, key hurdles to action have fallen away.

German Chancellor Angela Merkel, eying the carnage in Aleppo and the severity of the refugee crisis that has rocked Europe, is reportedly taking a tougher line.

After a call with Obama last week, the two leaders decried "barbarous Russian and Syrian regime airstrikes against eastern Aleppo," saying in unusually strong language that Moscow and Damascus bear a special responsibility to stop the fighting.

But none of the sanctions options under consideration is likely to directly end fighting in Aleppo.

Several officials poured cold water on the idea that Obama would reverse years of opposition to military action against the regime by approving air or cruise missile strikes against airfields or other targets.

"There are significant consequences for using US military force against the Assad regime," Earnest said. "I'd say the most important of those consequences that we should be mindful of is dragging the United States into another ground war in the Middle East."

Sending in troops "would have grave consequences for our national security," he added. "It would be expensive, it would put at risk more American lives and it's unclear how a conflict like that would end."

Obama was elected with a mandate to draw down the wars in Iraq and Afghanistan.

He has sent around 300 troops to Syria, focused on the battle against the Islamic State group, but has refused to plunge them into a civil war that is not deemed in America's strategic interest.

He has backed diplomacy instead as the only way out of the crisis.

But with just months left in office, the siege and bombardment of Syria's second city has put his reluctance to use force in Syria back under the spotlight, exposing deep unease within his administration.