"In January 2018 there had been almost 11,000 foreign fighters, about 70 percent of them are American, the others are from other countries," Abdel Mahdi told a weekly press briefing on Tuesday evening.

"In December, the numbers have been reduced to almost 8,000, and the American troops are around 6,000... maybe I am wrong by some hundreds."

Abdel Mahdi said that more than 12 months after the government declared victory over the Islamic State group in Iraq, the drawdown was accelerating.

"In recent months, the decrease has sped up and in the last two months there was a drop of 1,000 forces," he said.

US President Donald Trump has said that US troops will remain in Iraq after the withdrawal of all troops from Syria and will be available to take action against IS on the other side of the border if necessary.

US troop numbers in Iraq peaked at some 170,000 during the battle against Al-Qaeda and other insurgents that followed the US-led invasion of 2003.

Trump's predecessor Barack Obama ordered a withdrawal that was completed in 2011, but in 2014 ordered a new deployment as part of a US-led coalition battling IS, which had proclaimed a "caliphate" in large swathes of Iraq and Syria under its control.

IS is now confined to a shrinking enclave of just 15 square kilometres (under six square miles) in eastern Syria not far from the border where Kurdish-led forces have been engaged in a major offensive with coalition support since May last year.

In Iraq, the jihadists maintain sleeper cells in the cities and hideouts in sparsely populated desert and mountain areas from which they carry out periodic hit-and-run attacks, some of them deadly.