Turkey has seized a Syrian-flagged ship loaded with weapons, Turkish Prime Minister Recep Tayyip Erdogan has said.
He did not say where or when the vessel was stopped, but vowed to confiscate any arms shipments for Syria coming through Turkish waters or airspace.
Mr Erdogan has repeatedly criticised Syrian President Bashar al-Assad over his government's violent crackdown on street protesters.
Last week, Mr Erdogan said the era of oppressive dictators was over.
"Turkey has arrested a ship flying the Syrian flag and carrying weapons," Mr Erdogan said in New York where he attended the UN General Assembly, Turkey's Anataolia news agency reports.
"If in the future arms shipments (to Syria) are made by air or land, we will stop and seize them," he added.
News of this latest incident marks a hardening in relations between two formerly friendly countries, the BBC's Matthew Well in New York reports.
Ankara has grown increasingly impatient with Damascus' refusal to discuss political reforms, our correspondent says.
Earlier this week, Mr Erdogan told reporters that Turkey no longer had any contacts with the Syrian leadership and warned of sanctions.
Ankara also hosted a conference of Syria's opposition groups earlier this month.
Foreign reporters are not being allowed in to Syria so news organisations are largely dependent on unverified amateur video and reports by rights groups.
According to the UN, more than 2,200 people have been killed in Syria since anti-government protests began more than five months ago.
The government says hundreds of its personnel have been killed.
The government insists that it has a political reform programme in place, but opposition leaders say there can be no free and fair elections while the ruling Baath party is in power.
