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

The Pentagon confirmed about 50 military personnel were at Al-Asad air base to assess it for the possible deployment of 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 up to 3,100 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, which has seized control of swathes of Iraq and Syria, and some of Baghdad's forces who were hard-pressed by the jihadists fell back to Al-Asad air base.

The sprawling desert airfield was hub for US forces from 2003 until 2011, when it was transferred back to Iraqi control.

The 50 American military personnel assessing the base were not in evidence on Tuesday, but a slew of English-language signs and the colourful Easter decorations still hanging in a dining hall pointed to the former US presence.

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

Some of Anbar's powerful tribes are battling the Sunni extremist group, and have played an important role in keeping more of the province from falling.

Parliament speaker Salim al-Juburi visited Anbar tribal leaders at Al-Asad on Tuesday to "raise morale" and appeal for "weapons and equipment to face the danger of terrorism," he told AFP.

Sheikh Ashur Jabr Hamadi, one of the tribal leaders at Al-Asad, said the lack of ammunition was a problem, but the "most important thing we need is air cover".

Washington has forged an alliance of Western and Arab nations to take on IS, and launched a barrage of air strikes against it in Syria and Iraq.

Strikes on Friday were said to have targeted a gathering of IS leaders, but there has been no confirmation of reports IS chief Abu Bakr al-Baghdadi was killed or wounded.

US officials have insisted American troops will not 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 rebel forces battling President Bashar al-Assad, but Washington came under fire Tuesday for having a "confused" strategy.

"The coalition is fighting the symptom of the problem, which is (IS), without addressing the main cause, which is the (Syrian) regime," said Hadi al-Bahra, leader of the opposition National Coalition.

"The whole operation has been confused. Air strikes will not be able to win the battle against extremism. You have to defeat (IS) on the ground," he told The Guardian newspaper.

"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 "a significant contribution" to equip and train the moderate opposition.

Kurds fighting IS in Kobane made advances Tuesday in the south of the Syrian town bordering Turkey, said the Syrian Observatory for Human Rights.

Syrian Kurdish chief Saleh Muslim said his forces were advancing "street by street" and would "recapture the town in a very short time".

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

The UN is now pushing a plan for what envoy Staffan de Mistura calls a fighting "freeze" in limited areas.

Assad has said he is ready to consider such a plan for Syria's second city Aleppo, and de Mistura said Damascus had shown "constructive interest".

But the rebel Free Syrian Army effectively rejected the freeze, setting virtually impossible-to-achieve conditions.

The UN refugee agency said meanwhile it had been forced to slash the number of people it can help prepare for winter in Syria and Iraq for lack of funds.

Who's who at the Islamic State jihadist group
Baghdad (AFP) Nov 11, 2014 - Top figures with the Islamic State group, including possibly its chief, have been targeted in air strikes by the US-led coalition in Iraq.

Reports of IS leaders being wounded or killed are raising questions about how the jihadist group is organised in the territories under its control in Iraq and Syria.

"IS's (administrative) structure -- both at the provincial and transnational level -- gives all the trappings of a real state," Aymenn al-Tamimi, a fellow at the Middle East Forum, told AFP.

If IS leader Abu Bakr al-Baghdadi has in fact been killed or seriously wounded, it could spell trouble for the group. "The thing about the Islamic State is that a lot actually goes down to the persona of Baghdadi. If he actually got killed, I don't know who will replace him," Tamimi said.

Details about IS's organisational structure are murky, but its public statements as well as groups that track militant organisations and media reports point to the existence of some main elements:

-The Caliph: Abu Bakr al-Baghdadi heads both the militant group and its self-declared state encompassing parts of Iraq and Syria. His title of "caliph" is an attempt to revive a system of Islamic government last seen under the Ottoman Empire, and he has commanded Muslims around the world to obey him.

-Deputies: Abu Muslim al-Turkmani is said to be Baghdadi's deputy responsible for Iraq, where IS has overrun much of the Sunni Arab heartland since June, while Abu Ali al-Anbari oversees neighbouring Syria, where the group also holds significant territory.

-Military commander: Omar al-Shishani is a Chechen national sporting a distinctive, bushy orange beard who is portrayed as IS's top military commander.

-Spokesman: Abu Mohammed al-Adnani is the group's spokesman and one of its senior leaders. He has spoken in lengthy audio messages, but his face has not been shown in official IS media.

-Shura council: Said to be made up of senior IS figures who advise Baghdadi and carry out his orders. Some reports also indicate that Baghdadi has a separate cabinet.

-Other councils: There are reportedly other councils that deal with specific issues including military affairs, security, finance and media.

-Provinces: The group has divided up territory in Iraq and Syria that it holds (and some areas that it does not) into "wilayas," or states. They are headed by governors, and also have local military and administrative structures.