"We find the evidence to be strong that arms smuggling is continuing across the Syrian-Lebanese border," Ambassador Jeffrey Feltman told AFP, without giving any specific details.
"We are concerned by the reports and by the public statements by Hizbullah that Hizbullah has actively rearmed."
He added: "In our view this poses one of the biggest dangers to Lebanon and it is a violation of the spirit and the letter of a number of Security Council resolutions."
The Lebanese group, which is backed by Syria and Iran, is considered a terrorist organization by the United States. It had one minister in the Lebanese government until he and five other pro-Syrian ministers resigned last November.
Hizbullah, which claimed its resistance against Israel forced the Jewish state's 2000 withdrawal from south Lebanon, seized two Israeli soldiers in July 2006, sparking last year's devastating 34-day war with Israel.
This led subsequently to the reinforcement of the United Nations Interim Force in Lebanon (UNIFIL) and the deployment of the Lebanese army along the tense border zone with Israel for the first time in decades.
The group, however, was not disarmed and recently said it still had weapons that could reach Israel.
Feltman said there were several initiatives under discussion with the Lebanese government on how to put a stop to weapons smuggling.
He also rejected arguments that controlling Lebanon's border with Syria would amount to interfering in the country's sovereignty.
"Controlling borders is an assertion of sovereignty," he maintained.