Aleppo: Russia on Tuesday said it has sent an advanced missile system to the Syrian port of Tartus, as tensions escalate between it and the United States over the five-year conflict.
The announcement came after Washington said it was suspending talks with Moscow aimed at reviving a ceasefire deal over Russia's support for the regime of Syrian President Bashar al-Assad.
On the ground, Assad's forces advanced on rebels during intense street fighting in the opposition-held east of Aleppo city, which Russia has been accused of bombing indiscriminately including targeting its hospitals.
The UN rights chief called for action to halt the "ghastly avalanche of violence" unfolding in Syria's second city, which is reeling from some of the most brutal fighting in the conflict.
Russia, which has denied its strikes have hit hospitals, said it was deploying an S-300 missile system to Tartus on the Mediterranean coast.
"The S-300 is a purely defensive system and poses no threat to anyone," said defence ministry spokesman Igor Konashenkov.
"It's not clear why the placement of S-300 in Syria has caused such a stir among our western colleagues," he said in a statement.
As well as operating a naval facility in Tartus, Russia runs an air base outside the Syrian coastal city of Latakia, which currently houses warplanes used in its bombing campaign in support of ally Assad.
In August, a Russian official said Moscow was planning to expand into a permanent military facility its Hmeimim air base, which already has an S-400 air defence system, its most modern arsenal.
Washington announced late Monday that it would suspend joint efforts to reinstate a nationwide truce, accusing Moscow of abetting Assad's assault on Aleppo.
"Everybody's patience with Russia has run out," said White House spokesman Josh Earnest.
A US official said Secretary of State John Kerry was focused on finding a diplomatic solution, but his talks with Russian counterpart Sergei Lavrov on the crisis were over.
Kerry said on Tuesday the decision was one "we did not come to lightly".
"We are not abandoning the pursuit of peace, we are not going to leave the multilateral field, we are going to continue to try to find a way forward in order to end this
war," he said.
"People who are serious about making peace behave differently from the way Russia has chosen to behave," he added.