"If such an action has been taken, and we continue to analyze this issue, it would represent a failure by the parties in the region to honor UN Security Council Resolution 1701," State Department spokesman PJ Crowley said.
"And clearly it potentially puts Lebanon at significant risk."
Israeli President Shimon Peres Tuesday accused Syria of supplying the Scuds but a US official, speaking on condition of anonymity, said it was unclear if the transfers had yet taken place.
There was also disquiet in the White House, as President Barack Obama tries to engage Syria and unblock the impasse in peace talks between Israel and the Palestinians.
"We are obviously increasingly concerned about the sophisticated weaponry that is allegedly being transferred," White House spokesman Robert Gibbs said.
"We have expressed our concerns to those governments and believe that steps should be taken to reduce any risk and any danger," Gibbs said, apparently referring to the Israeli and Syrian governments.
"This is a... potentially destabilizing effect... the alarming effect that this has. We have expressed our great concern about [it]."
Another US official earlier told AFP that there was concern the missile sale was "under consideration, but it's unclear whether or not the missiles have been transferred."
According to Arab media and some think-tanks, Syria has been sending some of its own arsenal of Scuds to Lebanon, an allegation denied by Damascus.
Crowley said that Washington had been so concerned that US officials had brought up the alleged transfer in one of their regular meetings with the Syrian ambassador in Washington.
Obama in February nominated career diplomat Robert Ford as the first US ambassador to Syria in five years, seeking to energize his thwarted Middle East peace push.
If confirmed by the Senate, Ford would be the first US ambassador to Damascus since Washington recalled its envoy after Lebanon's former prime minister Rafiq Hariri was killed in February 2005 in a bombing blamed on Syria.
Crowley said that the administration was keen to get an ambassador in place partly so it could raise such concerns with Syria on a continual basis.
Peres told public radio Tuesday: "Syria claims it wants peace while at the same time it delivers Scuds to Hezbollah whose only goal is to threaten the state of Israel."
He made the comments just hours before flying to Paris, where he is expected to discuss the issue with French President Nicolas Sarkozy.
Israel's Deputy Defense Minister Matan Vilnai declined to go into details of the alleged Scud shipments, but said "Hezbollah's firing capacity has significantly improved."
Israel estimates Hezbollah's arsenal at some 40,000 rockets, a significant rise from the group's 14,000 rockets in 2006, when a 34-day conflict killed 1,200 Lebanese, mostly civilians, and more than 160 Israelis, mostly soldiers.
Hezbollah is blacklisted by Washington as a terrorist organization, although it is part of a Lebanese coalition government formed in November.
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