US troops on the ground in Iraq's Anbar as Islamic State campaign expands
Iraqi and US soldiers at a joint US-Iraqi military exercise on April 18, 2011, south of Baghdad (AFP Photo/Ahmad al-Rubaye)

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 jihadist 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 jihadists 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.

- 'Confused' US strategy slammed -

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 jihadists 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.

"And you have to deal with the main cause and source of extremism, which is the regime itself."

After meeting Bahra on Monday, British Foreign Secretary Philip Hammond promised London would make "a significant contribution" to equip and train the moderate opposition.

Kurds fighting the IS in Kobane made advances Tuesday in the south of the flashpoint Syrian town on the border with Turkey, a monitoring group said.

"The (Kurdish) People's Protection Units (YPG) recaptured streets and buildings in the south of Kobane, after a fierce battle against the IS that began yesterday (Monday) evening," said the Syrian Observatory for Human Rights.

The battle against IS has in recent months overshadowed the civil war in Syria, where more than 195,000 people have been killed since the start of an uprising in March 2011.

After the failure of previous peace bids, the United Nations is now pushing a plan for what UN envoy Staffan de Mistura has called a "freeze" in fighting in limited areas.

Assad said Monday he was ready to consider such a plan for the city of Aleppo, Syria's former economic hub that has been devastated by fighting.

Aleppo has been split into rebel- and army-held areas since a major insurgent offensive began there, with near-daily air raids targeting rebel-held districts and reportedly killing mostly civilians.

The situation in Syria was part of discussions between Obama and Russian President Vladimir Putin on the sidelines of an Asia-Pacific summit in China on Tuesday, the White House said.