MUNICH (AP) — Russia has proposed a March 1 ceasefire in Syria, U.S. officials say, 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 Wednesday. 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 weren’t authorized to speak publicly about private diplomatic discussions in the run-up to the Munich conference and demanded anonymity. One said the U.S. can’t accept Russia’s offer because opposition forces could suffer irreversible losses in northern and southern 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.

Secretary of State John Kerry, who arrived in Germany Wednesday, had talks planned late in the evening with U.N. peace envoy Staffan de Mistura and Adel al-Jubeir, the foreign minister of Saudi Arabia, a key backer of Syria’s rebel groups.

The Obama administration has been trying for months to clinch a ceasefire and pave the way for a transition government in Syria that would allow parties to the conflict to concentrate on defeating the threat posed by the Islamic State and the al-Qaida-linked Nusra Front.

But after having long demanded Assad’s ouster, the shift in the U.S. focus to combating terrorism has resulted in a confusing mix of priorities and a layered strategy in Syria that few understand, and even fewer see working. Beyond Russia, the administration has often struggled to keep its own allies such as Turkey and Saudi Arabia in line.