"Russia, Syria, and Iran are killing, or on their way to killing, thousands of innocent civilians in Idlib Province. Don't do it!" the president wrote on Twitter.
Sophie, Countess of Wessex, works with Angelina Jolie's Preventing Sexual Violence in Conflict Initiative.
As fewer Syrian refugees are allowed into the U.S., the conflict in Syria "continues to burn," said the vice president of policy and advocacy for the International Rescue Committee.
Recorded phone calls suggest around 215 Russian mercenaries were killed by U.S. artillery and airstrikes in a battle in Syria on the night of February 7.
President Donald Trump's foreign policy is creating a path for other countries to exert control over the Middle East, according to analysts.
As forces supported by the U.S. and Russia defeat ISIS across Syria, Secretary of State Rex Tillerson seeks Moscow's help to prevent a new conflict.
After being attacked, a coalition of forces backing the Syrian government and fighting the Islamic State militant group has warned the U.S. not to cross "red lines."
Jordanian commentators, including the military's former deputy chief of staff, rejected notions that the U.S., Jordan and allies were invading Syria by land.
Syrian and Iranian concerns regarding the U.S. and Jordan's nearby military drills were heightened when U.S. troops were seen with rebels in Syria.
The U.S. and Russia will once again work together to avoid mid-air collisions in Syria after a falling out between the nations last month.
Buses full of rebels began leaving the Barzeh district of Damascus as the Syrian government announced hopes for peace between factions.
The U.S.-backed, majority-Kurd Syrian Democratic Forces and the Russia-backed Syrian military are separately threatening ISIS's final major strongholds.
Syria's Bashar al-Assad said President Donald Trump 'has no policies' of his own and he only follows U.S. plans to advance political and economic interests.
The U.S. military killed 18 fighters of the allied Syrian Democratic Forces in a friendly fire incident in northern Syria.
The Kinetic Energy Projectile launches a tungsten-based missile at Mach 3 that delivers a "devastating" load of hot metal fragments.
Putin claims to have evidence that the rebels are planning chemical attacks that will be pinned on the Syrian regime.
Russia bolstered its support for Syrian President Bashar al-Assad in the face of U.S. military action.
Syria's war, now in its seventh year, has killed hundreds of thousands and drawn local and international forces into a deadly contest for power.