Syrian regime forces are advancing inside the opposition-held east of Aleppo city. In a "defensive" tactic, Russia has sent a missile system to the Syrian port of Tartus.
This is the latest move by the Syrian regime after it launched a Russia-backed offensive against opposition-held areas in Aleppo following the failure of a ceasefire brokered by pro-opposition US and Russia.
Regime tanks crossed the frontline in Aleppo for the first time in four years, Al Jazeera reported.
The Syrian Observatory for Human Rights (SOHR) said regime forces were "gradually advancing" in the city amid street battles.
Russia has been accused of attacking Aleppo indiscriminately, including bombing rebel-held hospitals as part of the major assault. Moscow on Tuesday said it deployed a S-300 missile system to Tartus on the Mediterranean coast, but that the move was "purely defensive."
Pentagon Press Secretary Peter Cook said the Russian missile system would not affect operations in the US-led air campaign against DAESH in northern Syria, and questioned why Moscow was making the move.
"Last I checked, the Russians said their primary goal was to fight extremism, ISIL and Nusra, in Syria," he said. Cook was referring to DAESH and the militant group formerly known as Al Nusra Front, which has changed its name to Jabhat Fateh al-Sham after claiming to disassociate itself from Al Qaeda.
"Neither one has an air force... So this is something we'll watch carefully. But it should be clear to the Russians and everybody else operating in Syria how seriously we take the safety of our air crews."
In addition to 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 the Assad regime.
"Everybody's patience with Russia has run out," said White House Press Secretary 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.
The US-Russia truce plan had envisioned an end to hostilities, increased aid deliveries, and eventual coordination in the fight against terrorism. The ceasefire collapsed after a week, with Russia blaming Washington for failing to convince rebels to distance themselves from "jihadists".
Aleppo aid convoy hit by ‘airstrike’
Analysis of satellite imagery of a deadly attack on an aid convoy in Syria last month showed it was an air strike, a UN expert said on Wednesday.
Some 20 people were killed in the attack on the UN and Syrian Arab Red Crescent convoy at Urem Al-Kubra near the northern city of Aleppo which destroyed 18 of 31 trucks, a warehouse and clinic.
"With our analysis, we determined it was an air strike and I think multiple other sources have said that as well," Lars Bromley, research adviser at UNOSAT, told a news briefing.
The United States blamed two Russian warplanes which it said were in the skies above the area at the time of the incident. Moscow denies the charge and says the convoy caught fire.
"For air strikes, what you are usually looking out for is the size of the crater that is visible and the type of crater," he said. A giant crater on the ground was caused "almost certainly (by) air dropped munitions" as opposed to artillery or mortars, he said.
UNOSAT, which reviews only commercially-available satellite images, has not been asked to share its analysis with the UN inquiry, but is prepared to do so, UNOSAT manager Einar Bjorgo said.