"And we're at a point now, where Hezbollah has far more rockets and missiles than most governments in the world, and this is obviously destabilising for the whole region and we're watching it very carefully," he told a news conference at the Pentagon.
Mr Barak also warned that Syria was arming Hezbollah with "weapons systems that can turn or disrupt the very delicate balance in Lebanon".
But he played down the chances of war over the alleged transfers.
"We do not intend to provoke any kind of major collision in Lebanon, or vis-a-vis Syria... but we are watching closely these developments and think that they do not contribute to stability in the region."
Syria has dismissed the American allegations on smuggling to Hezbollah
Mr Barak also chose not to repeat the allegation, made by Israeli President Shimon Peres last week, that Syria had been smuggling "Scud missiles to Hezbollah so that it can threaten peace".
A Hezbollah member of Lebanon's parliament told local media that the organisation would continue to rearm itself.
"Our choice was and remains to secure all the arms of resistance that we can," Hassan Fadlallah told the Arabic As-Safir newspaper.
But he said Hezbollah's armaments "do not compare" with the US or Israel's.
And another Hezbollah MP, Nawwar al-Sahili, told the BBC that the group had the right to buy whatever it needed from whomever it deemed appropriate in order to protect Lebanon.
Mr Sahili also said that Hezbollah chief Hassan Nasrallah - who recently said that if there was another war, Hezbollah would be able to hit Tel Aviv - never made empty threats.
Syria strongly denied the charge, saying it believed Israel aimed "through these claims to further strain the atmosphere in the region". Lebanon's prime minister said it seemed Israel was trying to find a pretext for a military strike.
Last week, Syria's most senior diplomat in Washington, Deputy Chief of Mission Zouheir Jabbour, was summoned by the state department to be warned about its "provocative behaviour".
A senior state department official later said the US would consider the "full range of tools" available to halt any smuggling of Scuds.
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