"We did inform the Russians through the deconfliction channel that are in place as you would expect we would," Kirby told MSNBC. "We take those obligations of ours seriously and they were informed prior to."
Russian Foreign Minister Sergey Lavrov in earlier remarks said the United States had informed the Russian military five minutes in advance about Thursday's airstrike against militia groups in eastern Syria.
The top Russian diplomat added that the US presence in Syria is illegal and violates all norms of international law.
Late on Thursday, the US Defence Department said that the US had targeted the infrastructure used by Iranian-backed militias – including Kait’ib Hezbollah and Kait’ib Sayyid al-Shuhada – in eastern Syria. Washington added that the strike was conducted in retaliation for the February 15 rocket attack on the Erbil Air Base in Iraqi Kurdistan.