Russia says it will unilaterally start using force against those violating the partial truce in Syria, if the US does not agree to joint rules by Tuesday.
A Russian general said delays in agreeing the rules were “unacceptable” and proposed holding an urgent meeting.
However, the US rejected the call, saying Russia’s concerns were already being handled in a constructive manner.
The cessation of hostilities has significantly reduced the violence in Syria since it began on 27 February.
However, both the Syrian government and opposition have complained of repeated violations.