The Pentagon spokesman also said that the USA continues "to effectively de-conflict coalition operations, however, we are not going to discuss the specifics of how we de-conflict operations in the highly congested and complex battlespace in Syria".
The Syrian government, with the help of Russian Federation and Iran, has gained the military upper hand against rebel groups, including some supported by Turkey, the USA and Gulf monarchies.
The agreement will remain in place for six months, a period which could be extended automatically if the three guarantor countries agree.
"The parties also exchanged views on the Syrian crisis, focusing on the decision to establish de-escalation zones, made on Thursday at the Astana meeting", the Kremlin press service added.
Russia, Iran and Turkey signed a ceasefire agreement yesterday without the Syrian rebel forces' approval and it will take effect tonight (Friday) at midnight.
But it was unclear how the deal differed from several previous failed cease-fires in which the Syrian air force continued to bomb rebel-held areas.
On Thursday, a memorandum was signed in Astana establishing four "safe zones" in Syria, where so-called "moderate opposition" fighters are expected to stay safe from airstrikes and keep terrorist groups out. State Department spokeswoman Heather Nauert said that the USA shares the opposition's concerns about Iran's role as a guarantor and about whether the government will really halt its attacks.
Deputy Defense Minister Aleksandr Fomin says the implementation of the safe zones in Syria may allow the war in Syria to finally stop.
Abo Zayd says rebel leaders are now meeting to come up with a formal stance on the Russian proposal.
It emerged Friday that the trilateral deal also seeks to ban USA aircraft from flying through the designated areas.
Along with Mr. Trump, the presidents of Russian Federation and Turkey have recently supported the idea of creating safe zones in Syria.
Russian official Alexander Lavrentyev suggested in peace talks on Friday that all military aircraft - including Russian and Turkish - would also be barred from the designated zones.
"It will be crucial to see this agreement actually improve the lives of Syrians", UN spokesman Stephane Dujarric said late on May 4.
The agreement is the latest attempt to reduce violence in the Arab country and would go into effect on Saturday.