Russia’s Foreign Ministry has called the U.S. threat to end joint cooperation in Syria “blackmail,” the RIA Novosti news agency reported Thursday.

U.S. Secretary of State John Kerry on Wednesday warned his Russian counterpart Sergei Lavrov that the U.S. could suspend bilateral engagement in Syria, including agreements reached earlier this month to establish a joint implementation center to coordinate airstrikes against the Islamic State and Nusra Front militants.

According to a U.S. government statement, Kerry “made it clear” that the U.S. holds Russia responsible for for the grave humanitarian situation in Aleppo and accused Russia, along with the Syrian regime it supports, of carrying out airstrikes on “hospitals, the water supply network, and other civilian infrastructure.”

Deputy Foreign Minister Sergei Ryabkov responded, saying Kerry’s remarks were “the politics of threats and blackmail” and called them an attempt to bring about resolutions beneficial to the U.S. and its “clients,” RIA reported.

He also claimed it was “impossible” to stabilize and deescalate the situation in Syria on such grounds, adding that Russia had “its practices and its principles.”

Ryabkov, in a later statement to the TASS news agency, accused the U.S. of not keeping to promises agreed in a recent cease-fire agreement and of “cynical threats toward those fighting terrorism in Syria.”

This undermining of the anti-terrorist operation in Syria, Ryabkov claimed, “can only be characterized as de-facto support of terrorists by the current U.S. administration.”

He also stated Russia’s continuing opposition to “unacceptable” week-long cease-fire agreements in Syria as proposed “for reasons known only to them” by the United States. This period, he claimed, allowed terrorists to fully rearm and regroup.

Russia fully favors a 48-hour cessation of hostilities, an offer, Ryabkov said, that is “still on the table.”

The Islamic State and Nusra Front are terrorist organizations banned in Russia.

Moscow Accuses Washington of 'Blackmail' and 'Supporting Terrorists' in Syria