A high-ranking defector's testimony details the regime's plan to target foreign journalists to silence independent reporting on the now seven-year-long conflict.
The Pentagon has denied responsibility for the attack.
With ISIS, Syria's universal enemy, largely defeated last year, international powers have begun to turn on each other in 2018.
The Syrian military has forced the insurgents of eastern Ghouta to either surrender or leave, but pro-Assad forces expected a fight with ISIS in the south.
Russia, Iran and Syria suspect that the U.S. is trying to split Syria and the region as a whole in order to take advantage of the unrest.
Russia's top diplomat argued that U.S. military presence in Syria was illegal and rebutted accusations over the poisoning of an ex-Soviet spy.
The U.S. and Russia have both contributed to the fight against ISIS, but their goals for postwar Syria differ.
A top Russian general reportedly intervened to stop an attack from the U.S., preventing a repeat from last month in which many Russians were killed.
Iraq and Syria declared victory over ISIS in late 2017, but the U.S. and Turkey have resisted calls to withdraw their forces.
Syria has been ravaged by a bloody civil war for the past seven years.
The U.S., Iran and Syria all have complicated relationships with Turkey, which sees the Syrian civil war as an opportunity to tackle Kurdish groups.
Russian Foreign Minister Sergey Lavrov warned that Moscow "made it clear" the U.S. would not be allowed to attack the Syrian military.
Tensions between local and international actors have led to nonstop violence and more than 500,000 deaths, but there are some signs of rehabilitation.
Less than two weeks after Russian President Vladimir Putin showed off advanced weapons, his officials are warning the U.S. against action in Syria.
The shifting dynamics of Syria's seven-year war have pitted the U.S. against NATO ally Turkey as well as Russia and Iran.
The Kurdish majority of the Pentagon-created Syrian Democratic Forces are leaving the U.S. battle against ISIS to fight a Turkish invasion.
China has joined Russia in backing Syria, partly because of the economic opportunities President Bashar al-Assad's victory offers.
North Korea and Syria have a history of working together against what they view as U.S. imperialism. In recent years, they've both seen success.
Russian Deputy Foreign Minister Sergey Ryabkov said U.S. military action against the Syrian military would be "unlawful."
One leading Russian senator said, "It would be good if the Americans, along with us, took part in liberating eastern Ghouta from the terrorists."
"Those who have to leave Syria are the ones who have not been permitted by the Syrian government," said one of the top advisers to Iran's supreme leader.