The Obama administration strongly warned Russia on Friday not to undermine peace efforts for Syria or upend the balance of power between Israel and its neighbors by supplying the Syrian regime with advanced antiaircraft weaponry.

Sending the S-300 missile to Syrian President Bashar al-Assad would prolong the civil war and perhaps widen it by imperiling Israel, Secretary of State John F. Kerry said amid conflicting reports about whether Russia has sent the missiles.

“We ask them again not to upset the balance within the region with respect to Israel and the weaponry that is being provided to Assad,” Kerry said. “It has a profoundly negative impact on the balance of interests and the stability of the region, and it does put Israel at risk.”

It was the strongest condemnation to date of the Russian sale, and it contrasts with the warm tone of cooperation between Kerry and Russian Foreign Minister Sergei Lavrov when the two met in Paris on Monday to align plans to draw the Syrian government and rebel representatives together for talks.

A proposed peace conference in Geneva is on hold while the anti-Assad rebels wrangle over internal divisions. The session will be a test of Russian commitment to a peaceful end to the war, Kerry said. He estimated the fighting has killed “upwards of 100,000.”

The United States backs the rebels, while Russia is a longtime ally and arms supplier to Assad. Although Assad has long wanted the advanced S-300, Russia has thus far sent less lethal weaponry, such as guns and ammunition.

Since the Syrian rebels don’t have warplanes, the S-300 missile system is seen chiefly as a threat to any Western or Arab nation that might try to impose a no-fly zone in Syria, or to Israeli warplanes that might attack Syrian chemical weapons depots. With the anti-Israeli militia Hezbollah now fighting in significant numbers alongside Assad’s forces, Israel is increasingly worried that the militants might seize chemical weapons for possible use against Israel.

“It is not, in our judgment, responsible because of the size of the weapon, the nature of the weapon and what it does to the region in terms of Israel’s security,” Kerry said Friday at a news conference with visiting German Foreign Minister Guido Westerwelle. “So we hope that they will refrain from that in the interests of making this peace process work.”

The United States has long promised to always ensure Israel’s “military edge” in a region filled with enemies. In practice that means the United States gives or sells advanced weapons and equipment to its close ally, and weighs sales to other nearby nations against Israeli concerns and interests.

“I would like to make this absolutely clear,” Westerwelle said in English. “We tell our Russian colleagues, don’t endanger the conference in Geneva. The delivery of weapons to the Assad regime is totally wrong.”

Kerry has said the United States and Russia have a common goal of ending the fighting and preventing the collapse of Syria and the rise of extremism.

But his remarks Friday suggests he is worried that Russia may be playing both sides.

“In Geneva, we will test who is serious,” Kerry said.

“Are the Russians serious about pushing to that? I believe they are. President Putin said they are. Sergei Lavrov has said it,” Kerry said. “Now, it is not helpful to have the S-300 transferred to the region while you are trying to organize this peace and create peace.”

Syrian opposition leaders are currently meeting in Turkey to ratify a new leadership slate, and the State Department welcomed the incremental progress they have made. The United States wanted a broader, secular leadership to dilute the influence of the Muslim Brotherhood.

The opposition leaders have not agreed to attend the U.S.- and Russian-sponsored peace meeting. Assad’s government has agreed to send representatives. The session could be held in June or July, under United Nations sponsorship.

“I’m not going to discuss consequences with respect to the opposition not coming because I’m convinced the opposition is going to come,” Kerry said.