The Syrian embassy has dismissed the allegations that it arms Hezbollah
The US has summoned Syria's most senior diplomat to review its "provocative behaviour" concerning the potential transfer of weapons to Hezbollah.
The state department told the deputy chief of mission in Washington, Zouheir Jabbour, that it condemned the arming of the Lebanese Shia Islamist group.
Particular concern was raised about the possible transfer of Scud missiles.
Last week, Israel's president accused Damascus of supplying the ballistic missiles to Hezbollah's military wing.
Hezbollah fought a 34-day conflict with Israel in 2006 during which more than 1,200 Lebanese people, mostly civilians, were killed. Some 160 Israeli people, most of whom were soldiers, also died.
UN Security Council resolution 1701, which ended the conflict, included an arms embargo on Lebanon, except for transfers authorised by the Lebanese government or UN.
But many analysts believe Hezbollah has since been rebuilding its arsenal with the help of its backers, Syria and Iran.
They say that if the group obtains ballistic missiles, it could alter the military balance in the region, putting all of Israel within reach.
In a statement after the meeting with Mr Jabbour on Monday, state department spokesman Gordon Duguid said: "This was the fourth occasion on which these concerns have been raised to the Syrian embassy in recent months, intended to further amplify our messages communicated to the Syrian government."
"The United States condemns in the strongest terms the transfer of any arms, and especially ballistic missile systems such as the Scud, from Syria to Hezbollah," he added.
Hezbollah fired rockets into Israel during the 2006 conflict
"The risk of miscalculation that could result from this type of escalation should make Syria reverse the ill-conceived policy it has pursued."
The Syrian embassy has previously dismissed the allegations and accused Israel of trying to divert attention from questions about its alleged nuclear programme. Israel neither confirms nor denies it has nuclear weapons.
Ties between Syria and the US had been tense since the killing in 2005 of Rafik Hariri, a former Lebanese prime minister. His assassination was blamed on Damascus - which denied any involvement - but Washington withdrew its ambassador shortly after.
The White House has recently been trying to engage diplomatically with President Bashar Assad and has appointed a new ambassador to Damascus, although his nomination is on hold in the US Senate.