Veteran military and diplomatic officials urged the US administration Thursday to keep all options on the table to fight Islamic militants, warning it would be a long, hard battle.

"I think we're up against in ISIL a very formidable and yet vulnerable enemy," former top military commander General James Mattis said, referring to the Islamic State group which is terrorizing parts of Iraq and Syria.

"It's going to be a long term fight, and an era of skirmishing over many years... it's not going to be over in one year or two years," he said.

Mattis, the former commander of the US Central Command, was joined by the ex-ambassador to Iraq, Ryan Crocker, to be quizzed by lawmakers a day after the House of Representatives voted to approve President Barack Obama's plan to equip and train Syrian rebels to fight the jihadists.

Obama has insisted there will be no US troops on the ground in Iraq or Syria to fight the Islamic State group, which has captured large swathes of territory in both countries.

While the US has sent in military advisors to Iraq to help defeat the militants, in Syria the plan is to beef up moderate Syrian rebels, already battling Syrian President Bashar al-Assad, to fight Islamic State militants.

But Mattis argued the vow not to send in US troops was a mistake, saying you should not "tell your adversary in advance what you are not going to do."

As the US moves to build a coalition of nations to fight the militants -- already said to number about 50 countries -- Mattis said the US should lead from the front.

"The fewer restrictions we place on own ourselves going into this, the more apt we are to see other nations give their full measure," Mattis told the House select intelligence committee.

As commander-in-chief, it would be up to Obama to decide when the US military should be deployed, but it would "send a reassuring note to the coalition... that on the military side we're all in," Mattis said.

"I'm not saying you have to commit them right now, but certainly on the military side don't pull them off the table," he added.

Crocker, who has also served in tough diplomatic posts such as Kabul and Beirut, agreed, pointing to other countries such as Yemen, Somalia and Afghanistan "where we face Al-Qaeda franchises."

He said while Al-Qaeda may be weakened, the remain a threat and urged "maximum flexibility" in the fight against the extremists.

"A phased approach to this current campaign is probably the right one. Don't take anything off the table... but don't put everything in the field up front either," Crocker said.

The US estimates that the Islamic State group has between 20,000 and 31,000 fighters, including many foreigners. But Mattis said it was made vulnerable as it was now trying to hold onto territory.

While the group's brutality may have attracted some followers it was also drawing allies to the US-led coalition.

Who's who in coalition to defeat Islamist extremists
Washington (AFP) Sept 18, 2014 - The United States is taking the lead in an international coalition aimed at repelling and defeating the Islamist extremists marauding across parts of Iraq and Syria.

Jihadists from the so-called "Islamic State," still referred to by US officials by their former acronyms ISIS or ISIL, have murdered thousands and declared a Muslim caliphate in the Middle East.

More than 50 countries -- mainly Western powers or Middle Eastern allies -- have committed to form a bulwark against the movement, the State Department said, not all the countries have been named.

Thirty of the participating states and organizations took part in a Paris conference last week and agreed to support the Iraqi government "by any means necessary" to fight the jihadists, including "appropriate military assistance".

Even though Iran and the United States both support Iraqi forces fighting IS, Tehran and Washington have both said they will not cooperate with the other. Syria is also a non-participant.

Military Assistance:

The United States has conducted 176 air strikes against IS targets in Iraq since August 8 and it has more than 800 military personnel to safeguard its Baghdad embassy and to assist Iraq's army.

It is talking with Iraq's new government about "accelerating efforts," including additional training and equipping of Iraqi Security Forces.

President Barack Obama will host a UN Security Council session September 24 on the threat of foreign fighters in Iraq and Syria.

Major partner Britain says it will ship �1.6 million ($2.6 million) worth of heavy machine guns on Wednesday to Kurdish forces fighting IS in Iraq. It is also considering providing training to the Kurds.

French President Francois Hollande announced Thursday it would join the United States in conducting air strikes in Iraq. Paris has already already begun reconnaissance flights over Iraq and sent weapons to Kurdish forces fighting IS.

But unlike the United States, Hollande was clear that France would not intervene in Syria, where IS militants also control swathes of territory.

Canada has authorized a 30-day deployment of "several dozen members of the Canadian armed forces" to help advise and assist Kurdish forces, as well as help airlifting in military supplies.

Australia's military transport planes have delivered weapons to Kurdish forces. Canberra insists it will not send combat troops to Iraq, but said last week it would deploy 600 troops to the United Arab Emirates, a regional Washington ally.

The Czech Republic has offered weapons to the Iraqi army, as well as ammunition and training to Kurdish forces.

Germany said it will provide military equipment and aid to Kurdish forces.

Albania, Italy and Poland have sent military equipment to Kurdish forces, and Warsaw also delivered tons of assistance to Christian and Yezidi refugees via its C-130 aircraft.

Estonia is donating one million artillery shells and Denmark's parliament has authorized its planes to resupply Iraqi and Kurdish forces.

The Philippines is prepared to "do its part" in an alliance, Foreign Affairs Secretary Albert del Rosario told AFP, but no further details were given.

Humanitarian aid:

Japanese Foreign Minister Fumio Kishida said Tuesday that Tokyo would support Iraq's "anti-terrorism fight" following inauguration of Prime Minister Haidar al-Abadi's new government in Baghdad.

Japan has committed $7.8 million in aid through the UN humanitarian office OCHA.

Switzerland has pledged more than $10 million in aid to OCHA. Norway committed at least $6 million, Denmark pledged $3.8 million, and Australia $4.6 million.

Finland, Hungary, Ireland, Italy, Luxembourg, New Zealand, South Korea and Spain have also pledged assistance.

Turkey is providing 100 truck loads of aid and a refugee camp near the Iraqi border.

The US, Britain, Canada, France and Australia have also either provided relief or conducted air drops.

The Gulf States:

A crucial element to the coalition are Arab and Gulf states, and at least 10 Arab states, including Saudi Arabia, have agreed to back the coalition.

Gulf oil monarchies, fearing jihadist threats at their doorsteps, had announced their opposition to IS in August.

Saudi Arabia, whose highest religious authority branded IS Islam's "number one enemy," has committed $500 million to the UN refugee agency, according to the State Department. Analysts said Riyadh's role would consist primarily of political and logistical support.

The same goes for Bahrain, home to the US Fifth Fleet. Kuwait, which has contributed $10 million in humanitarian assistance, could also lend use of its military facilities.

Oman has pledged "to play its part" in the battle against IS, without giving details.

Qatar -- which has furiously denies reports it has funded IS causes -- passed a law regulating charities that send money abroad or receive foreign financing. It could play a vital role. Its Al-Udeid Air Base hosts Centcom, the US military command responsible for the Middle East and Central Asia.

Regional US ally Egypt has said it will support Washington's efforts to repel the IS, but Cairo's involvement "must be under a UN mandate," an Egyptian foreign ministry official said.

The United Arab Emirates said it was prepared to join in a sustained effort to confront IS.