WASHINGTON (REUTERS) - The United States' top military officer said on Tuesday (Oct 16) that little progress had been made in dealing with the underlying conditions that have given rise to armed Islamist militants, even as military gains have been made against groups like the Islamic State in Iraq and Syria (ISIS).

"Little progress has been made in addressing the underlying conditions that lead to violent extremism," said Marine General Joseph Dunford, chairman of the Joint Chiefs of Staff.

US-backed forces and Iraqi militias liberated nearly all of the territory that ISIS once controlled in Iraq and Syria.

"Challenges remain in our political, our military, our intelligence, and our law enforcement cooperation despite the fact that we've had some positive trends and cooperation; clearly, there is much more to be done," said Gen Dunford, who was speaking during a conference on countering violent extremism.

One of the issues facing US-backed forces in Syria is the large number of foreign fighters being detained.

The US military says the Syrian Democratic Forces (SDF), an alliance of Kurdish and Arab fighters led by the Kurdish YPG militia, is holding about 700 foreign fighters and attempting to repatriate them to their home countries.

Speaking with reporters later on Tuesday, Gen Dunford said the detained foreign fighters came from 40 different countries and progress to try to repatriate them had been slow.

"The challenge has been there's not only political issues but... legitimate legal framework issues for addressing this in many of the countries," Gen Dunford said in a press conference.

He added that none of the more than 80 chiefs of defence from around the world at the conference were able to commit to doing something about the problem.

In a report earlier this year, the State Department warned that ISIS, Al-Qaeda and its affiliates have adapted by dispersing and becoming less vulnerable to military action after the US and its partners made "major strides" against the armed Islamist groups.

ISIS, Al-Qaeda and their affiliates "have proven to be resilient, determined and adaptable, and they have adjusted to heightened counter-terrorism pressure in Iraq, Syria, Afghanistan, Libya, Somalia, Yemen and elsewhere", the report said.

"Perhaps the greatest challenge facing us today is the danger of complacency," Gen Dunford said.

In a sign of the threat militant groups posed, the US military said on Tuesday that an air strike killed 60 militants from the Islamist al Shabaab group in Somalia.