The Obama administration has accused Russia of pursuing a "morally bankrupt" policy on Syria.
The State Department said Washington had "grave concern" Moscow is still supplying the regime of Bashar al-Assad with material that could be used to fight against rebels.
The reaction comes after Turkey ordered a Syrian passenger plane heading from Moscow to Damascus to land in Turkey on October 10.
Turkey's prime minister said on October 11 that the plane was carrying ammunition and military equipment for the Assad regime.
On October 12, Russian Foreign Minister Sergei Lavrov said the plane was legally carrying Russian radar parts for Syria.
Lavrov said the Russian company that shipped the cargo to Syria will demand that Turkey return it.
Lavrov did not name the company nor the cargo's final recipient inside Syria.
The U.S. State Department said Washington had no doubt "serious military equipment" was seized.
Lavrov did say the material was of "dual purpose," meaning it could have both military and civilians uses.
State Department spokeswoman Victoria Nuland acknowledged the shipments were likely legal.
But she said that was only because Russia and China have blocked efforts at the UN to slap sanctions, including an arms embargo, on the Assad regime.
"There are no Security Council sanctions on Syria because Security Council members Russia and China continue to block them," she said. "Everybody else on the Security Council is doing what it can unilaterally to ensure that the Assad regime is not getting support from the outside."
Turkey's ordering down of the plane comes amid rising tensions between Damascus and Ankara following cross-border shelling from Syria, including the killing of five Turkish civilians.
Inside Syria, rebels claimed they have seized an air defense base near the northern city of Aleppo, with footage posted online showing them in control of large missiles mounted on military trucks.
Rebels and government forces have for months been fighting for control of Aleppo, Syria's largest city and main commercial center.
The al-Ta'ana air base lies east of Aleppo.
Based on AP and AFP reporting