Speaking from Washington, Kerry warned that the planned delivery of S-300 air defence missiles to Damascus was "not helpful" as the US and Russia pressed on with their joint efforts to set up a peace conference, dubbed Geneva 2, to try to end the Syrian conflict.
Kerry said the delivery would have a "profoundly negative impact on the balance of interests and the stability of the region, and it does put Israel at risk".
German Foreign Minister Guido Westerwelle, in Washington for talks with Kerry, also appealed to Russia to consider.
"I would like to make this absolutely clear. We tell our Russian colleagues, don't endanger the conference in Geneva," he said.
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"The delivery of weapons to the Assad regime is totally wrong," he said.
In a television interview broadcast Thursday, Syrian President Bashar al-Assad appeared to imply that Russia had already started to deliver sophisticated S-300 missile systems under a contract with Syria.
Russian news agency Interfax reported that Moscow could supply 10 ultra-modern MiG-29 fighter jets to Syria under a possible contract being discussed with Damascus.
Syrian troops and Hezbollah allies meanwhile pressed ahead with their assault on the strategic town of Qusayr as hundreds of rebels reportedly broke through army lines to join the battle.
The Syrian opposition said yesterday that hundreds of rebel reinforcements, most of them close to the Muslim Brotherhood, had now reached Qusayr.
"Around 1,000 fighters from across Syria" had penetrated the town near the Lebanese border, the National Coalition's interim leader George Sabra told reporters in Istanbul.
The Coalition has appealed for the rescue of 1,000 civilians wounded in Qusayr, which Assad's forces have been trying to retake in an all-out offensive since May 19.
Thousands of people who have fled the besieged town are in dire need of aid, the UN's refugee agency said, as its tally for Syrians who have escaped their war-torn nation topped 1.6 million.
UNHCR spokesman Dan McNorton told reporters that at least 3,500 people -- mostly women and children -- had made it to nearby Hasiya.