The Lebanese navy has intercepted a Sierra Leone-registered ship carrying arms and ammunition reportedly destined for Syrian rebels, al Jazeera television said.
The ship's crew were detained after three crates full of machine guns, artillery shells and rocket-propelled grenades were found on board the Luftfallah II earlier this week.
Lebanese military prosecutor Saqr Saqr said an investigation was underway.
A security official told al Jazeera the arms were intended for members of the rebel Free Syrian Army, which wants to oust President Bashar al-Assad by force.
The ship was en route from Libya to the port of Tripoli in northern Lebanon. Several of the arms were labelled as Libyan.
It is now being held at Selaata, a small port some 50 kilometers north of the Lebanon's capital, Beirut.
The Syrian authorities have frequently complained that weapons are being smuggled from Lebanon into the country.
On Saturday, rebels in inflatable dinghies killed several securty officials in an attack on a Syrian military unit on the Mediterranean coast, state media reported.
The violence - believed to the first rebel attack from the sea - comes despite a UN-backed ceasefire in place since April 12.
Syria's official news agency Sana said security forces repelled an a "infiltration attempt" by a "terrorist group" in Latakia province.
Both sides have accused the other of violating the truce - part of a six-point peace plan brokered by UN and Arab League envoy Kofi Annan.
Russia has condemned what it called "barbaric" attacks designed to scuttle Annan's plan, saying the "terrorists" operating in Syria need a "decisive rebuff."
Moscow has vetoed two UN Security Council resolutions on Syria over what it says is a pro-rebel bias, but gave its full backing to Annan's plan.
"Attempts by the irreconcilable opposition to increase tension even more and incite violence cause particular alarm," the Foreign Ministry said in a statement.
Earlier this week, UN chief Ban Ki-moon again urged the Damascus government to comply with its commitments, but a state-owned newspaper accused him of "encouraging" terrorism in the country.
"[Ban] avoids talking about abuses by armed groups and focuses his blame solely on Syria, as usual. He encourages these groups to continue to commit more crimes and terrorist acts," an editorial in the Tishrin paper said on Saturday.
The UN says more than 9,000 people have been killed in clashes between the government and the opposition in Syria since the uprising against Assad began in March 2011.
The Syrian government insists it is combating "armed terrorist gangs" who are seeking to destabilize Syria.