The United States gave an extremely cautious welcome on Thursday to Russia and Turkey's plan to create safe zones to quell the Syrian civil war.
The State Department said Washington hopes the agreement will help stem the violence but expressed concern at Iran's involvement in the deal.
Earlier, Russia and Iran -- who support Bashar al-Assad's Syrian regime -- joined with rebel sympathizer Turkey to sign a safe zone pact.
The United States, which has supported some Syrian rebel groups, sent observers to the Astana talks, but was not a party to the agreement.
Some Syrian opposition members present in Astana opposed the agreement, and in particular the involvement of Assad's key ally Iran.
The United States also had its doubts about Tehran's role, and called into question the Islamic republic's commitment to peace.
"We continue to have concerns about the Astana agreement, including the involvement of Iran as a so-called 'guarantor'," State Department spokeswoman Heather Nauert said.
"Iran's activities in Syria have only contributed to the violence, not stopped it, and Iran's unquestioning support for the Assad regime has perpetuated the misery of ordinary Syrians."
But Nauert said that Washington "appreciated the efforts" of the other guarantors -- Russia and Turkey -- to defuse the situation.
"We nonetheless hope that this arrangement can contribute to a de-escalation of violence, end the suffering of the Syrian people, and set the stage for a political settlement of the conflict," she said.
"We look forward to continuing our dialogue with the Russian Federation on efforts to that can responsibly end the Syria conflict."
US Secretary of State Rex Tillerson is due to meet his Russian counterpart Sergei Lavrov next week on the sidelines of the Arctic Council in Fairbanks, Alaska.
Tillerson admits that there is little to no trust between Washington and Moscow and that relations are at a post Cold War low between the long-time great power rivals.
But he nevertheless hopes that common ground can be found on some issues, including on the need to de-escalate the civil war in Syria and refocus international military efforts on fighting Islamist extremists.