"We are concerned with the broadening nature of cooperation between Syria and Hezbollah," said a senior State Department official, who asked not to be named due to the sensitivity of the matter.
Damascus is "providing a wider array of missiles to Hezbollah," the diplomat added, but refused to accuse Syria of delivering the high-grade weapons to Hezbollah.
"As to the narrower question of do we have evidence of Scuds crossing into Lebanon, that's something ... to watch carefully," the official said.
President Barack Obama's administration has warned that "all options" were on the table if Syria is found to have supplied Scud missiles to Hezbollah, which would dramatically increase the Lebanese militia's ability to attack Israel.
Israeli officials have accused Syria of supplying Hezbollah with Scuds but Damascus has vehemently denied the charge.
'Really, really serious concerns'
Jeffrey Feltman, the assistant secretary of state for the Middle East, said Wednesday the United States would have "really, really serious concerns" if Syria had delivered such high-grade weapons to Hezbollah.
"If these reports turn out to be true, we're going to have to review the full range of tools that are available for us in order to make Syria reverse what would be an incendiary, provocative action," Feltman said.
On Monday, the most senior Syrian diplomat in Washington, Deputy Chief of Mission Zouheir Jabbour, was summoned to the State Department to review what the United States called "Syria's provocative behavior concerning the potential transfer of arms to Hezbollah."
Two days later, Syrian Foreign Minister Walid Moallem said it was regrettable that the United States had accepted Israel's accusations against Damascus as true.
The Scud allegations come as the United States steps up dialogue with Syria, and US lawmakers have seized upon the accusations to argue against any rapprochement between Washington and Damascus.
In February, Obama appointed the first US ambassador to Damascus in five years, Robert Ford, although the Senate has not yet confirmed him.
Syria, which Washington considers a state sponsor of terror, had a long military presence in Lebanon but withdrew its last troops in 2005 after an outcry following the assassination of pro-Western Lebanese Prime Minister Rafik Hariri, which has been widely blamed on Syria.