"Syria and Iran are providing Hezbollah with rockets and missiles of ever-increasing capability," Gates said at a joint news conference with his Israeli counterpart, Ehud Barak.
"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 destabilizing for the whole region and we're watching it very carefully."
Gates did not say if Syria was supplying Hezbollah with Scud missiles as Israel has alleged.
Barak voiced serious concern over Syria's support for Hezbollah but did not repeat the allegation that it was providing Scuds to the Lebanese Shiite militia.
Damascus has vehemently rejected the charge.
Barak said Syria was arming Hezbollah with "weapons systems that can turn or disrupt the very delicate balance in Lebanon."
He added: "We do not intend to provoke any kind of a major collision in Lebanon or vis-a-vis Syria."
Last week, 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."
Syrian Foreign Minister Walid Mouallem 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.
Both Gates and Barak said their countries agreed about the threat posed by Iran's nuclear program, with the Israeli defense minister endorsing Washington's efforts to secure fresh sanctions against Tehran.
But he said Israel wanted to see sanctions with a time limit.
"We expect the sanctions to be effective and to be limited in time so we will be able to judge …what kind of results stem from the sanctions regime," Barack said.
Amid recent tensions between the United States and Israel, both men stressed the strength of the bond between the two allies.
Gates said "our defense relationship is stronger than ever, to the mutual benefit of both nations."
The US defense secretary also sought to clarify how the American military views the effect of the stalled Middle East peace process.
Asked about recent congressional testimony from General David Petraeus, head of US Central Command, Gates said the United States viewed the lack of progress in the peace process as providing "political ammunition to our adversaries in the Middle East and in the region."
He said "progress in this arena will enable us not only to perhaps get others to support the peace process, but also support us in our efforts to try and impose effective sanctions against Iran."
But he said that "General Petraeus did not say that the lack of progress in the peace process is costing American lives."
