The United States and Britain said they are considering new sanctions to pressure the Syrian government and its ally, Russia, to end the bombardment of the besieged city of Aleppo.
British Foreign Secretary Boris Johnson and U.S. Secretary of State John Kerry made the warning on October 16 after briefing allies in London on the deteriorating situation in Syria.
Both also called for Russia and Iran -- key allies of Syrian President Bashar al-Assad -- to agree to a new cease-fire after a previous U.S.-Russia brokered cessation of hostilities collapsed last month.
For her part, the European Union’s foreign policy chief, Federica Mogherini, says the bloc is not considering sanctions against Russia for its actions in Syria.
"This has not been proposed by any member state," Mogherini said in Luxembourg on October 17 as she arrived for a meeting of EU foreign ministers expected to be dominated by the Syrian crisis.
However, she said the EU could extend sanctions against the Syrian regime as efforts to secure a cease-fire falter.
On October 15, Kerry met with Russian Foreign Minister Sergei Lavrov and other regional powers in the Swiss city of Lausanne, where the sides failed to agree on any concrete steps to halt the violence.
"There are a lot of measures that we're proposing, including extra measures on the regime and their supporters," Johnson told reporters. "These things will eventually come to bite the perpetrators of these crimes, and they should think about it now."
Johnson urged Moscow to "do the right thing by humanity and the people of Syria" by committing to a cease-fire.
He also called on the United States and Europe to make Russia "feel the consequences" of its military campaign in Syria.
Kerry confirmed Washington was considering additional sanctions over Syria but did not name Russia as a target.
"We are considering additional sanctions and we are also making clear that President [Barack] Obama has not taken any options off the table at this point in time," Kerry said.
Washington had suspended bilateral discussions with Moscow over Syria following two failed attempts at implementing a cease-fire.
Kerry described the situation in Aleppo as “the largest humanitarian disaster since World War II” and added that "crimes against humanity" were taking place there on a daily basis.
The Syrian government launched an offensive to capture rebel-held eastern Aleppo three weeks ago.
The United Nations says 275,000 civilians are still in the city and 8,000 rebels are opposing Syrian, Russian, and Iranian-backed forces.
Western powers have accused Russia and Syria of committing atrocities by bombing hospitals, killing civilians, and preventing medical evacuations.