MUNICH – Russia has proposed a March 1 ceasefire in Syria, U.S. officials said Wednesday, but Washington believes Moscow is giving itself and the Syrian government three weeks to try to crush moderate rebel groups.

The United States has countered with demands for the fighting to stop immediately, the officials said. Peace talks are supposed to resume by Feb. 25.

The talk of new ceasefire plans comes as the U.S., Russia and more than a dozen other countries meet in Munich to try to halt five years of civil war in the Arab country. The conflict has killed more than a quarter-million people, created Europe’s biggest refugee crisis since World War II and allowed the Islamic State to carve out its own territory across parts of Syria and neighboring Iraq.

Russia says it is supporting Syrian President Bashar Assad’s government as part of a counterterrorism campaign. But the West says the majority of its strikes are targeting moderate groups that are opposed to Assad and the Islamic State.

The most recent Russian-backed offensive, near Aleppo, prompted opposition groups to walk out of peace talks last month in Geneva, while forcing tens of thousands of civilians to flee toward the Turkish border.

The U.S. officials demanded anonymity. One said the U.S. can’t accept Russia’s offer because opposition forces could suffer irreversible losses in Syria before the ceasefire ever takes hold.

The officials said the U.S. counterproposal is simple: A ceasefire that is effective immediately and is accompanied by full humanitarian access to Syria’s besieged civilian centers.

Brett McGurk, the Obama administration’s point-man for defeating the Islamic State, said Russia’s Aleppo offensive was having the perverse effect of helping the extremists by drawing local fighters away from the battle against IS and to the war against Syria’s government.

“What Russia’s doing is directly enabling ISIL,” McGurk told the House Foreign Affairs Committee in Washington.

Secretary of State John Kerry emphasized Tuesday that U.S. officials “are not blind to what is happening.” He said the Aleppo battle makes it “much more difficult to be able to come to the table and to be able to have a serious conversation.”