Kerry said the US has expressed concerns about what such defensive systems in Syria would mean for Israel's security. He wouldn't address what the missiles might mean for Syria's civil war.
He spoke to reporters in Rome after the Wall Street Journal reported that Russia was preparing to sell the weapons to President Bashar Assad's regime.
Israeli officials said they have asked Russia to cancel the imminent sale to the Assad regime of advanced ground-to-air missile systems.
Russia rarely comments publicly on arms sales or transfers, and there has been no official word on the deal in Moscow.
Also Read
Even before Syria's 2011 uprising, the Israelis warned about a sale of S-300 batteries which can target manned planes, drones and incoming missiles. Moscow had held off on the deal under persistent US and Israeli pressure.
The S-300 would be a state-of-the-art upgrade for Syria's aging Soviet-supplied defense system, which was easily circumvented in 2007 when Israeli jets bombed a suspected nuclear reactor site along the Euphrates River in northeastern Syria.
The Wall Street Journal put the deal at USD 900 million for a package of four batteries, six launchers and 144 operational missiles. The missiles have a range of 125 miles, it reported, citing the Israeli-provided information, adding that the materiel would start arriving over the next three months.
Russia remains the Syrian government's most powerful international ally.
The enhanced Russian military support, if confirmed, would fly in the face of American claims that Moscow is demonstrating a new cooperativeness.
Moscow, with China, has repeatedly foiled Washington on Syria, blocking three UN Security Council resolutions against the Assad regime. It's unclear how its recent calculus has changed, even as US officials point to statements by Lavrov and other Russian officials showing less support for Assad's continued leadership.