Even so, Russia has sent signals it is hoping to gain support for the agreement from the United States despite their deep differences over the Syria war, now in its seventh year.
Foreign Minister Sergey V. Lavrov of Russia will visit with his American counterpart, Secretary of State Rex W. Tillerson, in Washington on Wednesday to discuss Syria and other issues, both sides announced on Monday.
Mr. Lavrov will be the highest-ranking Russian official to visit Washington since the Trump administration took office, and it will be his first trip there in years.
The de-escalation zones agreement, reached in Astana, Kazakhstan, on Thursday, is regarded as one of the more ambitious diplomatic undertakings by outside powers to halt the war, but it also has raised intense skepticism from insurgents and from some of their supporters, including the United States.
“Moscow has invested all of its cards in the Astana process,” Charles Lister, a senior fellow at the Middle East Institute in Washington, said in a briefing posted on its website. “Russia has a great deal to lose should this initiative fall apart, which makes acquiring a more committed U.S. statement of support extremely important.”
The State Department has expressed concern about the role of Iran in the agreement and the history of failed cease-fires in the war, which has claimed hundreds of thousands of lives and left millions of Syrians displaced.
But Defense Secretary Jim Mattis appeared to offer a muted expression of support on Monday, telling reporters during a visit to Denmark that the United States would closely examine the agreement.
“All wars eventually come to an end, and we’ve been looking for a long time how to bring this one to an end,” he told reporters. “So we’ll look at the proposal and see if it can work.”
Under the agreement, which is to last initially for six months, all combatants in the conflict are forbidden to use weapons in the de-escalation zones, including warplanes. The agreement also allows humanitarian aid to civilians in these areas.
The agreement does not apply to fighters loyal to the Islamic State or a Qaeda-linked group commonly known as the Nusra Front, which theoretically remains vulnerable to attack.
It is still unclear how the agreement might affect American airstrikes on Islamic State targets in Syria. A senior Russian diplomat, Aleksandr Lavrentiev, said on Friday that the agreement would effectively stop American warplanes from flying in Syria’s airspace. But a State Department spokesman, Edgar Vasquez, disputed that assertion, saying it “makes no sense.”
