"I do not rule it out," Bogdanov said when asked whether the United States could be a member of the working groups.
He added that the contacts were underway with the US side.
"Three guarantor states [Russia, Iran, Turkey] are involved," Bogdanov said.
"Our minister travelled to Washington, and we have contacts on other levels with our US partners," Bogdanov added.
The Kazakh capital of Astana has hosted four rounds of Syria peace talks so far, on January 23-24, February 15-16, March 14-15 and May 3-4. An agreement on the establishment of a joint monitoring group to track the ceasefire violations was reached and a memorandum on de-escalation zones was worked out. Four safe zones include the province of Idlib and seven other regions.