Turkey has accused Russia of directly supplying arms to Bashar al-Assad's Syrian government, after Turkish forces intercepted a Syrian plane en route from Moscow to Damascus, and forced it to land in Ankara.
Russia has denied that the plane was carrying military equipment - which Turkish Prime Minister Recep Tayipp Erdogan implied was a deliberate falsehood.
He told reporters from the Associated Press: "These were equipment and ammunition that were being sent from a Russian agency … to the Syrian defence ministry.
"Their examination is continuing and the necessary [action] will follow."
Russia has angrily accused Turkey of endangering the lives of its citizens when Turkish fighter jets forced the plane to land in Ankara on Wednesday evening.
The "illegal" cargo is said to include communication devices like radios and antennae and equipment "thought to be missile parts".
Turkish authorities spent hours conducting searches on board the passenger plane and "objectionable materials" were removed before the plane was allowed to leave, foreign minister Ahmet Davutoglu told Anatolia, Turkish state news agency.
A Syrian passenger plane is seen after it was forced to land at Ankara airport
The Airbus A320 airliner has a capacity of 180 passengers but only 30 people were on board when the plane was intercepted.
Turkey said it would not allow military equipment to be transferred through its airspace.
President Vladimir Putin has postponed his scheduled visit to Ankara following the incident, reported AFP.
"Neither weapons nor any systems or assembly parts for military equipment were or could have been on board the passenger plane," an unidentified high-ranking source from one of Russia's arms exporting agencies told Interfax.
"If there were a need to deliver military equipment or armaments to Syria, it would be done in a routine way rather than illegally, let alone by using a passenger plane," the source said.
Syrian rebel fighters hold up their guns as they shout slogans in a street in the northern city of Aleppo
Russia has supplied Syria with weapons in the past, and provided diplomatic support in the UN security council. Turkey has allowed limited weapons to be delivered across the border to the Syrian rebels.
Syria's Assad sought to pour oil on the troubled relations between Turkey and his country, saying they had "no reason to go to war" and describing the two countries as "brothers" in an interview with Turkish newspaper Aydinlik.
"In times like this, countries should correct their mistakes by talking to each other."