A large group of Islamic State (IS, formerly ISIS/ISIL) militants have been making massive advances in Deir ez-Zor province, cutting off a key air base supplying both anti-jihadist fighters and civilians in the region, RT's Lizzie Phelan reported from Syria.

"Frightening developments" is how Phelan has described the situation unfolding in the eastern Syrian province near the Iraqi border over the last few days. She said the terrorists had succeeded in cutting the last government-held patch of the Deir ez-Zor city in two, and severed a government-held military base from the rest of the city.

The airbase had been used by the Syrian government to deliver supplies to an estimated 120,000 civilians trapped in the city, which has been under the siege by the militants who had blocked all other delivery routes. Helicopters, which have been the only source of aid deliveries, including food and medical supplies, are now unable to land at the base, the RT crew reported.

Following reports of #ISIL attacks, @UN deeply concerned for the safety & protection of some 100,000 people in besieged #DeirezZor#Syria — OCHA Syria (@OCHA_Syria) January 17, 2017

The helicopters reportedly cannot deliver reinforcements to pro-government forces as well, which might enable IS to overrun the city and commit atrocities against its residents. People who have been living under constant IS shelling, told RT the attacks have been particularly bad over the last few days, calling it the most fierce offensive by the Islamic militants they've seen in months.

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Having spoken to civilians and pro-government fighters in the besieged areas, RT's reporter said people described their living conditions as being "extremely tough." With the lack of food, people are mostly surviving on canned food, she said, while a complete lack of fuel has forced people to mainly use bicycles. With the supporting airbase cut off, it's going to be even more difficult, Phelan reported.

It is believed the Islamists now involved in the Deir ez-Zor offensive came from the Iraqi city of Mosul, where they had been forced out by a US-led military operation against the terrorists. Last month, the Russian Defense Ministry warned that more than 5,000 terrorists had moved to Syria's Raqqa and Deir ez-Zor from Iraq's Mosul.

"What is very telling in the recent ISIS Deir ez-Zor offensive is that many of the groups who have led it are special groups of elite ISIS fighters who come from Iraq. So there is a direct link between what's happening in Mosul and the intense pressure that ISIS is under, which is translating on the battlefield in Syria," Danny Makki, a Syria analyst told RT.

He also shifted blame on Washington for not being "genuine in its fight against ISIS." There is "no doubt" the US-led coalition has seen convoys of IS terrorists crossing into Syria from Mosul and heading to Deir ez-Zor, he said, accusing the coalition of not having attacked the terrorists and allowing them regroup in Syria.