Officials in Moscow say they will contemplate a 48-hour cease-fire but nothing longer.
As the assault on Aleppo continues, the Obama administration sees few viable options.
Throughout the day, warplanes suspected to belong to both Syria and Russia pounded rebel-held areas of Aleppo.
The letter is the latest in a series of joint public statements by former officials.
Obama administration has said Russia is responsible for an airstrike on a humanitarian convoy.
The United States and other nations insisted the cease-fire is not dead, despite heavy fighting.
Russian and U.S. officials expressed doubt that truce can be salvaged amid latest flareup.
Secret police, hired killers and a former Chilean diplomat’s brazen murder in the streets of D.C.: The assassination of Orlando Letelier, as told by those who knew him and found his killers.
Assad government declares end to cease-fire, attacks Aleppo.
The senior Islamic State militant apparently oversaw the group’s information and propaganda activities.
