Turkish Prime Minister Recep Tayyip Erdogan says the cargo Turkey seized from a Syrian passenger plane that was forced to land in Ankara contained military equipment and ammunition destined for Syria's government.

Mr. Erdogan told reporters Thursday that a Russian supplier had provided the illicit cargo. He did not elaborate on where Turkey received the intelligence or who in Russia had provided the materials. Earlier, Syrian officials had denied the plane was carrying any military cargo.

Turkish military jets forced the plane to land late Wednesday in the Turkish capital on suspicion that it was carrying weapons from Russia to the Syrian government of President Bashar al-Assad. A crew member on the plane said Turkish authorities handcuffed them and made them lay on the ground when searching the plane.

The Syrian Air flight was allowed to complete its trip to Syria early Thursday, after Turkey confiscated what it called illicit cargo.

Syria responded strongly to the forced landing and cargo seizure Thursday, saying Turkey's decision was “hostile and reprehensible” and that it amounted to piracy.

Russia, a top ally of Mr. Assad, demanded an explanation from Turkey Thursday, saying its actions threatened the lives and safety of the passengers on board, which included 17 Russian citizens. Reuters reported that Russia's ambassador in Turkey was summoned to the Turkish foreign ministry on Thursday.

Turkish troops have repeatedly shelled Syrian military targets in recent days in response to Syrian artillery rounds that landed just inside Turkey.

Turkish military chief General Necdet Ozel said Wednesday that his forces will respond with “greater force” if Syrian shelling continues to spill across the border. He was speaking on a visit to the Turkish border village of Akcakale, where Syrian artillery killed five Turkish civilians last week.

Syria's President Assad has been fighting a 19-month uprising against his rule that has killed tens of thousands of people.