The US-led Operation Inherent Resolve is close to defeating the so-called Islamic State the "last area of real physical terrain", according to British Army Major General Christopher Ghika, who was speaking at a press briefing at the Pentagon.
The Operation, which is made up of 79 coalition countries including the UK, has carried out more than 100 air and artillery strikes in recent weeks.
Major General Ghika told reporters: "The coalition and our Syrian Democratic Forces (SDF) partners are fighting ISIS in the last are of real physical terrain which they hold in the Middle Euphrates River Valley around the town of Hajin.
"This is a hard fight, indeed, we have witnessed some of the most intense fighting since ISIS were defeated in Raqqa and Mosul."
Britain's contribution to the fight against IS, known as Op Shader, has been going for four years
Major General Ghika earlier praised the "immense progress" made by the Iraqi security force and the SDF over the last three years.
The two forces are backed by the coalition, with British troops involved in their training.
Operation Shader, the UK's contribution to the fight on IS or Daesh, is in its fourth year.
The UK remains the second largest contributor behind the US, with jets still flying near daily missions from their base at RAF Akrotiri in Cyprus.
In the last financial year, 2017 to 2018, the bill for fighting Daesh reached more than £583m.