"We expect the US side to continue strictly adhering to the line of cooperation with the Russian military in the interests of maintaining the ceasefire in Syria…talks or leaks regarding major supplies of weapons to militants under the infamous 'Plan B', including means to combat aircraft and artillery, are inappropriate," the ministry said in a statement.
Plan A presupposes the current cessation of hostilities between Syrian government forces and rebel groups, with the exception of terrorist networks operating in the country. The ceasefire took effect on February 27, and has been described as largely holding despite occasional violations.
On February 24, two days after Russia and the United States negotiated the ceasefire, White House spokesperson Josh Earnest said the United States was "not ruling out a Plan B" in case the truce did not hold. Moscow questioned US commitment to the cessation of hostilities with its contingency plan.
Earlier this week, media reports emerged claiming that the Central Intelligence Agency and its regional partners are preparing Plan B in Syria that includes delivery of various types of anti-aircraft weapons to Syrian rebels.