BEIRUT: Russian-backed Syrian regime forces inched closer Monday to a key stop on a vital Islamic State group supply line, as a twin offensive bore down on the jihadists' northern stronghold.
The advance comes as 17 civilians were killed in air raids on a popular market in eastern Syria on the first day of the holy Muslim month of Ramadan.
The UN, meanwhile, backtracked on its plan to move ahead with airdrops of humanitarian aid to Syria, saying it was focusing for now on security access for land convoys.
"Our main focus is on land delivery, given the challenges in terms of safety and logistics of air deliveries," UN spokesman Stephane Dujarric said.
In the battleground town of Tabqa, IS fighters are caught between Russian-backed regime forces pushing from the southwest and US-supported Kurdish and Arab fighters from the north.
The Syrian Democratic Forces (SDF), led by the Kurdish People's Protection Units (YPG), last week launched an operation against Tabqa and a nearby dam from the north of Raqa province.
But while they remain 60 kilometres (40 miles) north of Tabqa, government forces advanced on Monday, the Syrian Observatory for Human Rights said.
Regime fighters are now within 24 kilometres (15 miles) of Lake Assad, the key reservoir in the Euphrates Valley contained by the Tabqa Dam, said the Britain-based Observatory.
Observatory head Rami Abdel Rahman said regime forces were "reinforcing their positions" south of the town, and are less than 25 kilometres from Tabqa airport.
Vow of support
Russia last month floated a proposal for joint air strikes with the US against jihadists in Syria, but this was swiftly rejected.
However, a source close to the regime said "there is a joint operations room in Baghdad where the Iraqis and the Syrians are coordinating with the support of the Americans and the Russians".
Around Tabqa in particular, the source said, it would be "impossible" for the US and Russia to back their respective ground allies if they did not coordinate.
On Monday, Russia's Foreign Minister Sergei Lavrov vowed air support around Aleppo for troops loyal to Syria's President Bashar al-Assad.
"(The Americans) know that we will be actively supporting the Syrian army from the air to prevent terrorists from seizing territory," said Lavrov.
Washington "is asking us and Syrian leadership to delay air strikes" until opposition forces are separated from jihadists of IS and the Al-Qaeda-linked Al-Nusra Front, he said.
"We believe there has been more than enough time (for that)," Lavrov said, while adding "there won't be any surprises for the Americans".
In Washington, a State Department spokeswoman insisted US officials are in talks with groups on the ground to encourage moderate elements to separate themselves from Al-Nusra.
"We would reiterate that Russia and the Assad regime need to distinguish between the terrorists and parties to the cessation of hostilities," said Elizabeth Trudeau.
Two years after declaring a fundamentalist "caliphate", IS is coming under mounting international pressure.
IS is also under attack in Aleppo province, after SDF fighters crossed the Euphrates near the border with Turkey and pushed west towards jihadist-held Manbij.
Analysts suspect the SDF's operational focus on Manbij may explain its minimal progress towards Tabqa.
Manbij lies at the heart of IS-held territory along the border that US commanders regard as the principal entry point for foreign fighters and funds.
Tabqa is further along that route, closer to Raqa city.
SDF forces are now two kilometres away from Manbij to the south, six kilometres from the south and seven from the east, according to the Syrian Observatory.
The forces are trying to surround the city from 3 fronts and leave one open route from the west for IS fighters to withdraw, the Observatory chief said.
Raqa 'last to fall'
Syria's conflict has evolved into a complex war involving foreign powers since starting in 2011 with the brutal repression of anti-government protests.
Peace talks to end the five-year war -- which has killed more than 280,000 people and displaced millions -- have stalled and a related ceasefire is in tatters.
On Monday, 17 civilians, among them eight children, were killed in air strikes on a market in Al-Asharah, an IS-held town in eastern Deir Ezzor province.
While fighting for Tabqa and Manbij intensifies, it appears the battle for Raqa city -- which would be a much more symbolic victory -- has taken a backseat.
The US-backed SDF alliance's offensive north of the IS stronghold last month started amid much fanfare.
But Henman said "Raqa will likely be one of, if not the last Islamic State bastion to fall in Syria".