"The border between Lebanon and Syria remains highly porous," Assistant Secretary of State David Welch told a Congressional panel.
Welch said Washington agreed with a recent report by UN Secretary General Ban Ki-moon asserting "serious breaches" of the arms embargo imposed under a UN Security Council resolution which ended last year’s war between Israel and Hezbollah.
"It is clear in (Ban’s) judgment, and it is clear in our own independent (judgment) that Hezbollah continues to rearm and we can see no other source for such assistance than Syria or Iran," said Welch, the top State Department official for the Middle East.
"We are encouraging the Lebanese army and UNIFIL to take a more assertive role in stopping smuggling," he said, referring to an expanded United Nations peacekeeping force deployed in Lebanon following the July-August war.
Welch said Hezbollah, which is also a political movement in Lebanon, of campaigning to overthrow the elected government of Prime Minister Fuad Siniora, with Syria’s backing.
He said one aim was to thwart the establishment of a UN-backed special tribunal to investigate the February 2005 assassination of former Lebanese prime minister Rafiq Hariri.
Hariri was a strong opponent of Syria’s long domination of Lebanon and many believe Damascus was behind his murder.
Welch said that if Lebanon is unable to formally endorse the creation of the special tribunal due to opposition from Hezbollah and other pro-Syrian parties in parliament, the US could back unilateral action by the UN.
"If the Lebanese government is unable to approve the agreement, the (Security) Council may need to consider other mechanisms for establishing the Tribunal, including under UN Security Council Chapter VII authority," he said.
Chapter VII of the UN Charter gives the Security Council the power to impose mandatory actions on member states.
Welch was speaking after the Security Council on Tuesday asked Ban to send an independent mission to investigate reports of illegal arms movements across the Lebanese-Syrian border.
The council expressed its "serious concern at mounting information by Israel and another state" of arms smuggling across the border in violation of UN resolution 1701.
Syria has denied that arms are making their way over the border into Lebanon and warned against any moves to station international troops along the frontier in Lebanon, which Damascus occupied for 30 years until being forced by popular protests to withdraw following Hariri’s assassination.
Ban is due to visit Syria next Tuesday.