The secretary of State warned Russia not to dig in its support of Syria's Bashar al-Assad.
The letter shows that some Democrats are feeling frustrated by the lack of progress.
The White House pushed back against reports that the program has been halted.
"We are not where we should be at this point," said the former CIA director.
The two countries could soon be operating against ISIS in Syrian airspace.
Russia is also increasing its military flights to the country, according to U.S. officials.
The first class disintegrated after they deployed into Syria.
The Pentagon called it an "important development" in the fight against ISIS.
The Nusra Front released seven U.S.-backed rebels after they were kidnapped last month.
It's unclear whether the United States will try to get them back.
The flights will likely increase the number of coalition strikes in Syria.
Michael Ratney has "impressed" John Kerry, the secretary of State said.
The Pentagon denies the program itself has been suspended.
The two sides will hold "deconfliction discussions" in the fight against ISIS.
Sen. Chris Murphy called news of the rebels handing over equipment to al-Nusra Front "disturbing."
The second group of rebels entered "successfully," U.S. Central Command said.
Pentagon officials say signs point to Russia establishing a forward air operating base in Syria.
"This is a splendid opportunity for the global jihadists," the presidential candidate said.
Sen. Tim Kaine (D-Va.) has urged Congress to pass a new AUMF.
The rebels had pledged to fight only ISIS with their American training.
The program is speeding up after a shaky start, the Pentagon said.
Pentagon says strikes could target groups other than ISIS if rebels are threatened.
The announcement represents an increased role for NATO.
Establishing the zones could draw the U.S. into a war with Syria.