"We condemn the continued supply of Russian weapons to the regime, and this includes all class of weapons, and we've been clear throughout and very direct with the Russian government about that," State Department spokesman Patrick Ventrell told reporters after Moscow announced it would deliver an advanced guided S-300 anti-aircraft missile system to President Bashar Assad's regime after the European Union ended its embargo on providing arms to Syrian rebels.
"We've seen how the regime uses those arms," Ventrell said.
The long-range S-300 surface-to-air missile system, called the SA-10 Grumble by NATO, could enable the Syrian army to shoot down attacking aircraft from rebel-allied nations, military experts say. It would also bring the missiles within range of civilian aircraft.
Ventrell said the EU's decision to end the embargo yet abide by a 60-day waiting period before shipping any arms to rebels because of possible Syrian peace talks was welcomed by Washington.
"It certainly sends a message to the Assad regime," he said.
He also said arming Syria's opposition was different from arming the regime.
"Clearly they are people who are defending themselves in the face of an enormous onslaught and a despicable onslaught of violence," Ventrell said.
White House spokesman Jay Carney said the Kremlin's decision to supply the airspace-controlling S-300s would hurt the possibility of a diplomatic end to the 26-month-old war that has taken more than 80,000 lives.
"We've made clear in the past and made clear again our firm belief that providing arms to the Assad regime does not bring us closer to the political transition that Syria deserves," Carney told reporters as diplomatic fears grew of an escalating proxy war in the region if Geneva, Switzerland, peace talks, now envisioned for July rather than June, were to fail.
At the same time, Israel indicated it would not stand by if the Russian weapons are delivered to Syria.
"As far as we are concerned, that is a threat," Israeli Defense Minister Moshe Yaalon told reporters Tuesday.
"At this stage, I can't say there is an escalation. The shipments have not been sent on their way yet. And I hope that they will not be sent," he said.
"If, God forbid, they do reach Syria, we will know what to do," he added.
An Israeli strike on Russian shipments to Syria would represent a sharp escalation. Until now, Israel has only launched strikes against shipments of advanced weapons it believed were intended for transfer to the Lebanese militia group Hezbollah.
Hezbollah has publicly embraced its military support of the Assad regime. At least two rockets launched from Syria landed in Lebanon Tuesday afternoon, a security official said.
The escalating rhetoric came a day after U.S. Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov met in Paris to promote the Geneva peace conference, which they envisioned would bring the Assad regime and rebels together.
Moscow says the Assad regime is willing to attend but says Iran, a key backer of the Assad military, must be included. Washington opposes this condition.
"We insist on Iran's participation in Syrian affairs and the Syrian conference mainly because all the countries that have influence on the various Syrian groups should be represented there, without exception," Lavrov said Tuesday.
The main Syrian opposition coalition hasn't said if it will formally attend, and many rebel fighters say it is too late for a negotiated solution.
Concerning the EU decision, Lavrov said it was the West that was sabotaging peace efforts, not Moscow, adding arming rebels could violate international law.
"A whole range of actions ... are serving to undermine the idea of calling a conference," he said.
The EU decision to allow arms shipments to Syrian rebels was "illegitimate," he said.
"To seriously discuss at an official level the theme of delivering or not delivering weapons to non-state parties contradicts all norms of international law," he said.
By contrast, Moscow cast its determination to deliver the S-300 system as a "stabilizing factor" Russian NATO envoy Alexander Grushko said was "in total compliance with international law."
"We will fulfill the signed contracts," Grushko said of the weapons the International Assessment and Strategy Center think tank in Virginia regards as one of the most lethal anti-aircraft missile systems in service or under development.
Russian Deputy Foreign Minister Sergei Ryabkov said Moscow believed providing the S-300s would "go a long way to restraining some hotheads from exploring scenarios in which this conflict could be given an international character with participation of outside forces."
France rejected Moscow's "hothead" suggestion.
"The decision to lift the embargo isn't bellicose," French Foreign Ministry spokesman Philippe Lalliot said.