Russia has proposed to the U.S.-led coalition that they stage joint airstrikes on Syrian rebels, including militant Islamist group Nusra Front, who are not observing a ceasefire, Russian Defence Minister Sergei Shoigu said on Friday.
Such action would begin as of May 25 and be coordinated with the Syrian government, he told a Defence Ministry meeting broadcast on state television, adding that Moscow reserved the right to stage strikes unilaterally.
The Pentagon said it had not been formally notified of the proposal by the Russian defense ministry and stressed that it was not collaborating with Russia.
"We've not received (the proposal) and I've seen the same reports you have," said Pentagon spokesman Captain Jeff Davis.
"I can only tell you from where I sit and where we sit today, that we do not collaborate or coordinate with the Russians on any operations in Syria."
Davis refused to speculate about any potential change in that position.
Washington has consistently refused to join any operation that is coordinated with the Syrian government, as has been the case with Russia's campaign of airstrikes that began in September last year.
Shoigu said joint airstrikes should also target convoys carrying weapons and ammunition crossing into Syria from Turkey.
While Russia supports the government of President Bashar al-Assad, the United States - along with its allies in the West and the Gulf - back rebels trying to overthrow him.
However, both sides oppose the al Qaeda-linked Nusra Front, which was not included in a ceasefire deal which has failed to prevent widespread violence.
Shoigu said the proposed joint airstrikes would help the stalled peace process.
"We believe the adoption of these measures will allow a transition to a peaceful process to be achieved in the entire territory of Syria," he said. "Of course, these measures have been coordinated with the leadership of the Syrian Arab Republic."
Shoigu said discussions with U.S. military experts based in Jordan and other counterparts in Geneva had begun on Thursday.
But he added: "We reserve the right to start from May 25 unilateral strikes on units of international terrorist groups and illegal armed groups which have not joined the ceasefire."
(Reporting by Dmitry Solovyov; additional reporting by Phil Stewart in Washington; Editing by David Stamp and Alistair Bell)