Weapons are being smuggled in both directions between Lebanon and Syria, the United Nations has said.
"Based on information that we have, there are reasons to believe that there is a flow of arms both ways - from Lebanon into Syria and from Syria into Lebanon," Terje Roed-Larsen, the UN special envoy to the Middle East, told reporters after briefing the 15-member UN Security Council on Tuesday about events in Lebanon.
"We do not have independent observers for this, but we are basing our reporting on information we are receiving from a variety of sources," he said.
According to Roed-Larsen's briefing notes, he told the council UN Secretary-General Ban Ki-moon had raised the issue of cross-border arms transfers with Lebanese officials during a recent visit to Beirut, urging them to improve border control.
Lebanese authorities seized 60,000 rounds of ammunition hidden in two cars on an Italian container ship docked at the northern Lebanese port of Tripoli, a security source said earlier on Tuesday. Tripoli, a mainly Sunni Muslim city, has seen regular protests in support of the uprising against President Bashar al-Assad.
In late April, Lebanese authorities seized a large consignment of Libyan weapons including rocket-propelled grenades and heavy calibre ammunition from a ship intercepted in the Mediterranean.
Syrian diplomats allege other ships have been intercepted carrying weapons to Syrian opposition fighters.
Roed-Larsen, the UN special representative on Security Council resolution 1159, passed in 2005 and calling for Syria to withdraw its forces from Lebanon, sees growing dangers in the Middle East.
"What we see across the region is a dance of death at the brink of the abyss of war," he said.
"If you look at the situation in Syria, which is maybe the most likely that might spill over, then in a way it reminds us of the situation in Lebanon and in its neighbouring countries in the 1970s. And this is what I fear," said the envoy, a veteran diplomat in the Middle East.
Roed-Larsen said there was complete "uncertainty" over what will happen and that it was vital for the UN Security Council to be ready to take action.