"There are a variety of things that have been considered, including the kinds of financial penalties that the United States has been able to apply in coordination with our allies in a variety of situations," Earnest told reporters. "I certainly wouldn't rule out something like that [sanctions] in the future."
The spokesman acknowledged, however, that the international community has seen no sign that existing financial sanctions have caused Russia to change its strategy in Syria.
He also said the United States and its allies would have pursued a military solution in Syria by now if one could be found.
"The ultimate solution here is a diplomatic one," Earnest asserted. "Russia, in particular, has been resistant to engaging constructively in pursuit of this solution."
Since the start of the civil war in Syria in 2011, Russia and Iran have recognized Assad as the country’s sole legitimate authority. Moscow and Tehran continue to pursue efforts to back Assad’s government, which is fighting terrorist groups such as the Islamic State and Jabhat Fateh al-Sham (previously known as the Nusra Front), both of which are banned in many countries, including Russia.