BAGHDAD - A team of US troops was on the ground in Iraq’s frontline Anbar province Tuesday as Washington steps up efforts to help Iraqi forces battle the Islamic State militant group.

The Pentagon confirmed that about 50 military personnel were at Al-Asad air base to prepare the way for a larger contingent of advisers and trainers to assist Iraqi security forces. President Barack Obama has announced plans to double the number of American troops in Iraq to 3,000 as US-led efforts against the militants enter what he called a “new phase”.

Parts of mainly-Sunni Anbar province have become a stronghold for IS and Iraqi forces have been on the retreat in recent weeks, falling back to the Asad air base. The desert airfield was a sprawling hub for American troops and aircraft during the 2003 to 2011 occupation of Iraq.

A string of battlefield defeats for Iraqi forces has led to warnings that Anbar, which stretches from borders with Syria, Jordan and Saudi Arabia to the western approach to Baghdad, could fall entirely.

Sunni extremist group IS has seized control of large parts of Iraq and Syria, declaring an Islamic “caliphate” in June and committing widespread atrocities.

Its influence has steadily grown, with Egypt’s deadliest militant group Ansar Beit al-Maqdis on Monday pledging allegiance to IS. Washington has forged an alliance of Western and Arab nations to take on the group and launched a barrage of air strikes in Syria and Iraq on IS positions.

One of the strikes on Friday was reported to have hit a gathering of IS leaders but there has been no confirmation of reports that IS chief Abu Bakr al-Baghdadi was wounded or killed.

US officials have insisted the mission will not see American troops engage in combat and are instead pushing for local forces to tackle IS on the ground.

For Syria the United States has approved plans to train 5,000 recruits from among moderate rebel forces battling President Bashar al-Assad, but Washington came under fire Tuesday for having a “confused” strategy. The leader of the moderate Syrian opposition, Hadi al-Bahra, said no strategy to target the militants would work as long as Assad remains in power.

“The coalition is fighting the symptom of the problem, which is ISIS, without addressing the main cause, which is the regime,” Bahra told The Guardian newspaper, using another name for IS.

“The whole operation has been confused. Air strikes will not be able to win the battle against extremism. You have to defeat ISIS on the ground,” said Bahra, who heads the Syrian National Coalition.

Meanwhile, the UN refugee agency said UNHCR on Tuesday that about 13.6 million people, equivalent to the population of London, have been displaced by conflicts in Syria and Iraq, many without food or shelter as winter starts.

Amin Awad, UNHCR’s director for the Middle East and North Africa, said the world was becoming numb to the refugees’ needs. “Now when we talk about a million people displaced over two months, or 500,000 overnight, the world is just not responding,” he told reporters in Geneva.

The 13.6 million include 7.2 million displaced within Syria - an increase from a long-held UN estimate of 6.5 million, as well as 3.3 million Syrian refugees abroad, 1.9 million displaced in Iraq and 190,000 who have left to seek safety.

The vast majority of Syrian refugees have gone to Lebanon, Jordan, Iraq and Turkey, which Awad said “are putting us all to shame” with their support for homeless Syrian families.