Secretary-General Kofi Annan, in a report to the UN Security Council on Wednesday, said the "illegal transfer of arms and people" over the Syrian-Lebanese border undermined Beirut's efforts to control its territory.
The 15-page report, prepared by UN special envoy Terje Roed Larsen, implied but did not directly say the Damascus government was supplying weapons to Palestinian groups in Lebanon that have their headquarters in Syria.
Syria's UN envoy, Fayssal Mekdad, said: "If you go to Lebanon you can find arms anywhere. We don't allow any export of weapons."
Hours before the report was issued, Lebanese troops and tanks encircled military bases run by pro-Syrian Palestinian fighters, witnesses and security sources in Beirut said.
The army set up checkpoints at Sultan Yacoub in the eastern Bekaa Valley, where the Syrian-backed Popular Front for the Liberation of Palestine-General Command runs a tunnel network dug into the hills.
Annan's report was a response to Security Council resolution 1559 in September 2004 that called for Syria to withdraw all troops and intelligence agents from Lebanon and for the disarmament of militias in Lebanon. This would include Palestinian groups and the Hizb Allah fighters, who dominate the south.
The document said recent reports showed "an increasing influx of weaponry and personnel from Syria to some of these groups". It said Syria acknowledged that arms and people were being smuggled over the border, "albeit in both directions".
Many blame Syria for the killing of
Rafiq al-Hariri on 14 February
Last week a UN investigating team led by German prosecutor Detlev Mehlis blamed Syrian security officials and their Lebanese allies of organising the murder of former Lebanese prime minister Rafiq al-Hariri on 14 February.
Mehlis said Syria was obstructing the investigation into the killing, which led to protests and international pressure resulting in Syria pulling its troops out of Lebanon after a 29-year presence.
A draft UN resolution, sponsored by the United States, France and Britain, demands Syria detain suspects Mehlis wants to question and calls for consideration of economic sanctions if Damascus does not comply.