Kurdish fighters, who earlier this week captured Tal Abyad, a key Syrian town on the Turkish frontier, after a few short days of intense fighting, are now vowing to clear northern Syria of Islamic extremists and have already begun advancing on villages 50 kilometers to the north of Raqqa.

According to opposition activists with the network Raqqa Is Being Slaughtered Silently, the Islamic State is, "preparing for military operations north of Raqqa city." The activists added: "They are digging trenches, building shelters and opening a new road."

The activists say mosque preachers have told residents to hoard food to withstand a siege.

Kurds' advance

The quick advance in recent days by Kurdish fighters of the YPG, People's Protection Units, has shaken up widespread Western military assumptions that pushing back the Islamic State, also known as ISIL, from key Syrian strongholds would take months.

The Kurdish offensive supported by some Arab rebel battalions has benefited from significant U.S.-led coalition airstrikes. A large number of U.S. airstrikes the past two weeks have been launched to assist the advancing Kurdish forces near Raqqa, Hasakah and Kobani.

While many YPG fighters say the coalition airstrikes were key to their success, some Syrian Kurdish activists say that don't fully explain their victory at Tal Abyad.

"Some will say without U.S. airstrikes the Kurds would not have succeeded. I will say to them even when U.S airstrikes helped the Iraqi army, they still failed and lost ground,"said activist Kovan Direj. He attributes the Kurdish advance in part to unity among Kurdish factions in Syria.

The speedy loss of Tal Abyad, the hub for a crucial supply route from Turkey for the Islamic State, suggests that the Islamic extremists were ill-prepared for the offensive and may be under-manned, say analysts.

Rapid demise

The quick fall of Tal Abyad may also be explained by the diversion of IS forces elsewhere in northern Syria. The group has been pressing an offensive to the northwest of Raqqa in an effort to seize a key border crossing at Azaz held by rebels from the Western-backed Free Syrian Army and Islamist brigades.

"The tables have been turned," a European defense attache based in Turkey told VOA. "ISIL has been smart with outflanking movements and quick to mount surprise attacks in unexpected places – this is what the Kurdish are now doing to the extremists." He spoke on the condition of anonymity as he is not authorized to speak to the media.

The jihadi offensive on Azaz has stalled but IS has started to impose an economic blockade on rebel-controlled northern Syria seeking to weaken the armed opposition and facilitate the bid to capture Azaz, say rebel commanders.

The jihadis are blocking fuel produced in their territory in eastern Syria as well as flour from getting to the rebel-dominated countryside around the cities of Aleppo and Idlib. Traders have been ordered to stop trucking diesel and refined oil into rebel-held areas in northern Syria on threat of being beheaded, if they ignore the order.

Domino effect

The blockade is affecting more than only rebel fighters. The interdicting of fuel is deepening the humanitarian crisis in northern Syria, disrupting everything from agriculture to the working of medical clinics, who rely on diesel to run their generators. Bakeries, water supplies and aid deliveries, which rely on local transportation networks once inside Syria, are also being badly impacted.

In northern Hama Province, health workers have declared a suspension of hospital and clinic work and activists say a dozen babies died at a medical facility in Maarat al-Numan because incubators couldn't be run.

IS has used its control of major oil facilities to generate cash and the group has traded fuel and oil with the Assad government as well as with rebel groups. But it has also cut off supplies when it serves the group's military purposes.

In a statement via aTwitter feed, Jaish al Fata, or the Army of Conquest, a mainly Islamist rebel alliance, said, using the Arab acronym for IS: "Daesh is not allowing Fuel in Aleppo."

According to opposition activist Yusuf Eissa, "Aleppo and Idlib [are] without diesel or gasoline."

Fuel blockade

The economic blockade – especially the fuel shortages – likely will force more civilians to flee to southern Turkey.

The fuel blockade is worsening the already desperate plight of hospitals and clinics in rebel-held areas. Medical workers claim the Assad government has been targeting clinics with barrel-bomb air raids.

On Monday a hospital in Busra in Dara'a governorate was destroyed after being hit by 10 barrel bombs – one of 10 hospitals attacked the past month.

Last week, a hospital supported by the international charity Médecins Sans Frontières/Doctors Without Borders (MSF) in Aleppo governorate, in northern Syria was hit by a barrel-bomb.

"We call on the warring parties to respect civilians, health facilities and medical staff according to humanitarian law," said Carlos Francisco, MSF's head of mission for Syria. He added: "These new attacks on medical infrastructures are unacceptable."