Moscow suggests that the US-led coalition fighting against Islamic State in Iraq and Syria should fly joint missions with Russia in Syria, the defense minister said.

“Taking such a step would help the progress of the peace settlement in Syria. Of course such measures have been agreed with the Syrian Arab Republic. Yesterday we started negotiating these measures with our colleagues in Oman and Geneva,” Defense Minister Sergey Shoigu told journalists, referring to a US center for peace negotiations based in the Jordanian capital and the city in Switzerland, where UN-backed peace negotiations are under way.

“We suggest to the US… starting on May 25, joint action of the Russian Air Forces and the US-led coalition forces to plan and conduct strikes against the Al-Nusra Front, which does not support the ceasefire, as well as against convoys of arms and fighters crossing the Syrian-Turkish border,” he said.

He suggested that the US and Russia should redouble their efforts to have moderate rebel groups pulled out from territories controlled by the terrorists, so that those rebels wouldn’t face Russian airstrikes.

Shoigu warned that, starting from next Wednesday, Russia would feel free to use its warplanes to attack any group that failed to back the ceasefire.

“Starting May 25, we reserve the right to unilaterally conduct airstrikes against forces of the international terrorist organizations and militant groups who didn’t join the truce,” the Russian minister warned.

He said that conducting joint missions would help eliminate the risk of inadvertently targeting civilians.

The suggestion puts the US in a difficult situation from a legal and public relations standpoint. Washington for years has been treating Syrian President Bashar Assad as an illegitimate figure, rather than Syria’s head of state. The airstrikes it conducts in Syria are illegal because the US has neither a mandate from the UN Security Council nor an invitation from Damascus to use force in a sovereign nation’s territory.

Moscow, for its part, was called upon by the Syrian government to help its army fight against terrorist forces. Joint Russian-US missions would technically require legal permission from Damascus to Washington, and asking for one would be a great embarrassment for the Obama administration and a serious blow for the Democratic Party, which would be exposed to Republican criticism in an election year.

The US and Russia are jointly backing the peace negotiations in Geneva. A ceasefire declared in Syria is part of the process, but it is not observed by some militant groups in Syria, including the terrorist organizations Islamic State and Al-Nusra Front. Attacks on government forces and rebel groups, which pledged to observe the truce, happen on regular basis in the country, and on some occasions provoke violent responses from those attacked.