The U.S. said that "serious military equipment" was found on board a passenger plane forced down by Turkish fighter jets during a flight between Moscow and Damascus last week and that it has spoken to Russia about it.
U.S. State Department spokeswoman Victoria Nuland said U.S. diplomats have spoken with Russians both in Washington and Moscow about the cargo confiscated from the Syrian airline by Turkish authorities at the Ankara airport on Oct. 11.
"We have been in contact with the Russians," Nuland told reporters in Washington on Wednesday.
"As you know, we were pretty definitive publicly about our grave concern that this kind of activity continues, particularly by a Security Council member," she said, according to a transcript on the State Department's website.
Turkey has not given a public account of what precisely was found, while Russian officials have said that the cargo included radar parts that had dual civilian and military use but were completely legal.
But Nuland indicated that the cargo was more serious than what the Russians had suggested.
"We've had a pretty comprehensive account from the Turkish side of precisely what they found," she said. "But I'm going to leave it to them to share in public what they found. … We have no doubt that this was serious military equipment.
The plane incident has cast a chill on Russia's relations with Turkey, but Ankara, worried about the months of violent civil unrest in neighboring Syria, has dismissed the idea that the consequences will be long term.
President Vladimir Putin appeared to bristle over the plane incident on Tuesday, telling a state arms trade commission that no country could restrict Russia's sales of weapons.
"Only sanctions imposed by the UN Security Council can serve as a basis for limiting weapons supplies," Putin said. "In all other cases, nobody can use any pretext to dictate to Russia on how it should trade and with whom."
Efforts by some Security Council members to impose sanctions have been blocked by Russia and China.
Russia's policy line was reaffirmed by Deputy Prime Minister Dmitry Rogozin on Thursday. "No one can ever complain about Russia in this respect; weapons export controls in our country are stricter than in many other countries," Rogozin said, according to Interfax.
Russian officials, in turn, have accused Western countries of supplying weapons to the Syrian rebel army.