US Secretary of State John Kerry said Friday that Russia's planned delivery of S-300 air defense missiles to Syria is "not helpful" for efforts to convene a peace conference.
Kerry's comments at a news conference with German Foreign Minister Guido Westerwelle came ahead of a preparatory meeting in Geneva next week on a US-Russian bid for talks to end Syria's two-year-old civil war.
"In Geneva, we will test who is serious. Are Russians serious about pushing for that? I believe they are. President (Vladimir) Putin said they are, (Foreign Minister) 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," he added.
"It is not helpful to have a lot of other ammunition and other supplies overtly going in not just from the Russians -- and they are supplying that kind of thing -- but also from the Iranians and Hezbollah."
Westerwelle expressed similar concern about the move to supply anti-aircraft missiles to President Bashar al-Assad's embattled regime.
"I would like to make this absolutely clear. We tell our Russian colleagues, don't endanger the conference in Geneva. The delivery of weapons to the Assad regime is totally wrong," he said.
Assad suggested this week that his government has already received the advanced air defense missiles, but on Friday, Russian media reported they had not yet been delivered.
Russia vowed to supply the missiles -- which would complicate any effort by Western countries to intervene militarily in the conflict -- after the European Union lifted a ban on providing arms to Syrian rebels.
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Israel has voiced concern about the missiles and warned it will act to prevent them from falling into the hands of Hezbollah or other armed groups.
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.
"And 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 added.
The Assad regime's regional allies are meanwhile drawing closer, with Lebanon's Iranian-backed Hezbollah militia sending fighters to help regime forces retake the strategic town of Qusayr near the Lebanese border.
Syria is Iran's closest Arab ally and has long served as a key conduit for arms shipments to Hezbollah. The three view themselves as an axis of "resistance" to Israel and the West.
"The Iranians have said they welcome this conference," Kerry said.
"Well, if they do, they need to show it in other ways than sending their forces across the border -- being the only nation in the world to have their fighters on the ground in an organized, state-supported way."
The United States hopes that with Russian support it can convene talks leading to a political solution to the bloody stalemate in Syria, which threatens to ignite unrest in neighboring countries.
But in an interview with Hezbollah's Al-Manar television broadcast Thursday, Assad said he was "very confident" of victory against the two-year-old uprising, in which more than 94,000 people have been killed.
The Syrian rebels meanwhile remain deeply divided, and have vowed to boycott all peace talks until Hezbollah withdraws from the country.