What is stalling the fight against IS in east Syria?

The Islamic State group has fiercely defended its last holdout in eastern Syria against a more than two-month military offensive by a Kurdish-Arab alliance backed by a US-led coalition.

After the Syrian Democratic Forces alliance successfully expelled the jihadists from other parts of war-torn Syria, why is this latest battle in the far east taking so long?

Who is inside the holdout?

Hundreds of fighters are believed to be inside the pocket, which lies in the oil-rich province of Deir Ezzor near the Iraqi border.

The US-led coalition has estimated some 2,000 fighters to be present in the holdout, which includes the towns of Hajin, Sousa and Al-Shaafa.

According to SDF commanders on the front line, a great number of those combatants are likely to be non-Syrians.

The SDF has said it believes "major leaders" of IS are hiding inside the pocket, where most front line commanders are Iraqis.

Nicholas Heras, an analyst at the Center for a New American Security, says "a mini army of several hundred fighters... has been assembled by IS there, including some of its best snipers."

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