It is thought to be the first rebel assault from the sea. Separately, Lebanon says its navy has seized weapons destined for the rebels.

Clashes between security forces and deserting troops left heavy casualties near Damascus and Aleppo, reports say.

The violence comes despite a shaky ceasefire in force since 12 April.

On Thursday UN Secretary General Ban Ki-moon warned that Syria's government was "in contravention" of a UN- and Arab League-backed peace plan.

Clashes

Saturday's violence came after the Lebanese navy said it had found and confiscated three containers full of arms and ammunition bound for the rebels.

The ship, the Lutfallah II, is reported to have begun its voyage from Libya, stopped off in Alexandria in Egypt, and then headed for the port of Tripoli in northern Lebanon before it was intercepted.

The BBC's Jim Muir in Beirut says it is believed the consignment was destined for the rebels in Syria, with whom the new Libyan regime strongly sympathises.

Tripoli in north Lebanon is a hotbed of support for the Syrian opposition, and the authorities in Damascus have frequently complained about arms being smuggled from the areas into the country, our correspondent says.

The dinghy attack reportedly took place further north, about 30km (19 miles) from the border with Turkey.

Syria's official news agency Sana said a military unit had foiled a "terrorist attempt" to infiltrate the country overnight by boat in Latakia province.

"An official source told a Sana reporter that members of the military unit clashed with the terrorists who were boarding inflatable boats, forcing them to flee," the agency said.

"The source stated that the clash led to the martyrdom and injuries of a number of [members of the] military unit." Sana said it was not clear how many rebels had been killed "as they attacked the military unit at night".

The fighting north of Damascus broke out after a group of soldiers defected to the rebels and were pursued by forces loyal to President Bashar al-Assad into the village of Bakha, activists said.

One account said four rebels and six civilians had been killed, but the British-based Syrian Observatory for Human Rights said all those who died were army defectors.

Similar clashes were reported in the village of Burj al-Salaam near one of the presidential palaces close to the coastal city of Latakia, after a group of soldiers deserted there.

None of violence could be independently verified because of government restrictions on the media.

More observers

The UN currently has about 15 observers in Syria monitoring a shaky ceasefire, which came into force on 12 April, and hopes to have the full advance team of 30 in place by Monday.

Violence has been continuing despite the truce.

On Friday an explosion in the centre of the Damascus killed at least 10 people and wounded 20 others, state media said. Activist organisations accused the regime itself of carrying out the attack.

Mr Ban has demanded that Damascus comply with the peace plan brokered by international peace envoy Kofi Annan without delay.

The Security Council has approved the deployment of up to 300 monitors. Norwegian Maj Gen Robert Mood, who is to lead the team, was heading to Damascus on Saturday, reports said.

Our correspondent says he must be wondering how much of a ceasefire there is left for his team to monitor.

Annan's six-point peace plan

1. Syrian-led political process to address the aspirations and concerns of the Syrian people

2. UN-supervised cessation of armed violence in all its forms by all parties to protect civilians

3. All parties to ensure provision of humanitarian assistance to all areas affected by the fighting, and implement a daily two-hour humanitarian pause

4. Authorities to intensify the pace and scale of release of arbitrarily detained persons

5. Authorities to ensure freedom of movement throughout the country for journalists

6. Authorities to respect freedom of association and the right to demonstrate peacefully