The United States and Germany have warned Russia that arming President Bashar al-Assad's forces would hurt efforts to bring Syria's warring parties together for peace talks.
Secretary of State John Kerry said Russian plans to send a sophisticated air defence system to President Assad also put Israel's security at risk.
He was speaking after talks with his German counterpart Guido Westerwelle.
The US and Russia are pressing for a peace conference in Geneva next month between Syria's government and opposition.
Mr Westerwelle called on Mr Assad to "stop the violence and come to the negotiating table" and told Russia that sending the S-300 missiles to the Syrian president "is totally wrong."
American woman killed fighting in Syria
An American woman has been killed in Syria as she fought with opposition forces against President Assad's regime.
Syria's state-run television showed pictures of a damaged car with broken windows,
It showed a passport and a driving license bearing the name of Nicole Mansfield of Flint, Michigan.
The woman's aunt in the United States said the Federal Bureau of Investigation had informed her of the death of her niece.
Monica Mansfield Speelman said she was devastated at the news.
She said Ms Mansfield, a single mother had converted to Islam about five years ago but that she did not know when her niece had travelled to Syria.
“I didn't think she would stoop that low to go over there and try to harm anybody,” she said.
A US State Department official, speaking on condition of anonymity, had earlier confirmed that Washington was working via the Czech Republic mission in Syria to get more information