Russian Federation has formally announced that delivery of its feared S-300 surface-to-air missile system to the Syrian government is now complete.
USA military spokesman Col. Sean Ryan told reporters at the Pentagon on Tuesday that the Iranian missiles "potentially jeopardize the forces on the ground that are actually fighting (IS) and puts them in danger".
Pressed about her earlier statement, Nauert was asked whether the confirmation of the S-300s would be "a "sort of serious escalation" or a serious escalation".
A Russian lawmaker this week said that 112 Russian military personnel have been killed so far in Syria since it started actively intervening in the conflict in 2015. Within three months, the Syrian specialists will be trained in the use of s-300 systems, Interfax reports.
Russian Foreign Minister Sergei Lavrov announced the deliveries had started during a United Nations press conference.
Fifteen Russian servicemen died when their Il-20 plane was accidentally downed by Syrian forces during an Israeli air strike in Latakia province.
Moscow said Israel's F-16 planes carrying out strikes on Latakia province were using it as "cover" which resulted in the deadliest case of friendly fire since Russian Federation intervened in the Syrian conflict in 2015.
In addition to the Russian and Syrian air forces, warplanes from the US-led coalition fighting IS have also been carrying out bombing raids on Syria since September 2014.
The Russian plane's downing threatened the close security ties between Russia and Israel.
The Robertron statement looks odd if one takes into account repeated USA accusations against Russian forces operating in the Middle East and in Syria particularly.
Israeli Prime Minister Benjamin Netanyahu has criticized Russia's decision as "irresponsible", saying Tel Aviv "will continue to do what it has to do to defend itself".
Syrian Foreign Minister Walid Muallem has praised Iran's missile attack on the positions of Daesh in Syria.
Israel says it has carried out more than 200 attacks against Iranian targets in Syria over the past 18 months. "If they are used against us, we will act against them".