US-backed fighters edged further into the Islamic State (IS) group stronghold of Manbij on Saturday, threatening a key staging post on the militants’ lifeline from Turkey to their "caliphate" in Syria and neighbouring Iraq.
If it succeeds, the offensive on Manbij -- backed by intense air strikes by a US-led coalition -- would mark the most significant victory against IS group for the Syrian Democratic Forces alliance.
In neighbouring Iraq, elite counter-terrorist forces and police were poised to assault the last IS-held neighbourhood of the militants’ once emblematic bastion of Fallujah in a further blow to their prestige.
In Manbij, SDF fighters overran a key road junction in the south of the city after capturing nearby grain silos overnight, the Syrian Observatory for Human Rights said.
"The grain silos overlook more than half of Manbij. SDF fighters can climb to the top and monitor the city," Observatory chief Rami Abdel Rahman said.
The Raqa Revolutionaries Brigades -- one of the Arab components of the Kurdish-dominated alliance -- confirmed that SDF forces had seized the silos and pushed into the city.
The Mills Roundabout lies less than two kilometres (one mile) from the city centre.
The Observatory said IS and the SDF were locked in intense street fighting as the militants tried to defend their positions.
Captured by IS group in 2014, Manbij was a key transit point for foreign fighters and funds, as well as a trafficking hub for oil, antiquities and other plundered goods.
Across the frontier in Fallujah, security forces were poised to assault Jolan, the last neighbourhood still held by IS.
Tens of thousands of people fled the fighting, with many camped out in the open in the summer heat.
"Dozens of families are still without tents or any form of shelter inside the camps, living in miserable conditions. The majority are elderly people, women and children," the Norwegian Refugee Council said.
The SDF launched its offensive to take Manbij on May 31, driving across the Euphrates River from the east with military advice from some 200 US special forces troops.
IS has thrown large numbers of fighters into the battle, losing 463, according to the Observatory. The SDF has lost at least 89.
The militants have taken some 900 Kurdish civilians hostage in areas under their control west of the city, according to the Observatory and Kurdish officials.
Manbij lies in the eastern plains of Aleppo province, which has become a battleground between an array of competing armed groups, including Al-Qaeda, non-militants rebels and government forces, as well as the SDF and IS.
Aleppo was once the country's commercial hub but now lies divided between government forces in the west and rebels in the east.
A two-day truce brokered by Moscow and Washington earlier this month in a bid to rescue a wider ceasefire expired without renewal and Russia warned it would press ahead with its air campaign, saying the rebels had failed to deliver on promises to break ranks with Al-Qaeda loyalists.
On Saturday, Russian and Syrian warplanes pounded rebel-held areas in and around the city in support of a regime offensive on the rebels' sole remaining supply route, the Observatory said.
The Castello Road has been repeatedly hit by air strikes but residents said the bombing had intensified in recent days.
An AFP correspondent in the rebel-held east of the city said the latest strikes lasted throughout the night.
"In the past two days, my kids and I haven't been able to sleep all night because of the huge blasts, the likes of which we haven't heard before," said 38-year-old shopkeeper Abu Ahmad.
A father of three, Abu Ahmad owns a small convenience store in east Aleppo.
"We haven't been able to get any products or produce for the shop over the past two days because no one can use the (Castello) road," he said.
The Observatory said government forces were also fighting rebels in northern neighbourhoods of the city in a bid to halt rocket fire on government-held districts.
The renewed government offensive around Aleppo comes a day after another key ally, Lebanese militant group Hezbollah, pledged to send more fighters to join the "greatest battle" of the war.
More than 280,000 people have been killed since the conflict began with anti-government protests in 2011.
A prominent Syrian activist and journalist died in a Turkish hospital overnight after being seriously wounded in a bomb blast in Aleppo last week.
Facebook pages managed by fellow activists said Khaled al-Issa, a Syrian photographer in his 20s, died in Antakya.
Journalist Hadi al-Abdullah, who was wounded in the same bombing that some activists have blamed on Al-Qaeda, was in stable condition in hospital.