Western powers are coming under mounting pressure to do more to confront Islamic State (IS or ISIS) in its stronghold in Syria, as the heavily armed militants edged closer to taking an important air base that would cement their domination over a swath of the country's north, the Guardian reports.

As U.S. aircraft continued to pound the Islamist militants in northern Iraq, the Obama administration was studying a range of options for pressuring ISIS in Syria, primarily through training "moderate" Syrian rebels as a proxy force, with air strikes as a possible backup.

Leaders in Washington and London are adamant they will not collaborate with the regime of Bashar al-Assad in tackling their common enemy, and on Friday, Aug 22, the Pentagon insisted that it had yet to decide on whether to expand the U.S. air war into Syria, the Guardian says.

But ISIS has demonstrated its rampant authority in northern Syria in recent days, with the brazen murder of the U.S. hostage James Foley and a series of attacks on towns and villages in the north, including the vital airbase at Taqba, where it has surrounded a detachment of Syrian army soldiers. It now holds a swath of territory in Syria and Iraq that is larger than the UK and home to at least four million people.

"The Islamic State is now the most capable military power in the Middle East outside Israel," a senior regional diplomat said on Friday.

U.S. officials have conceded that 93 air strikes in Iraq that have checked the ISIS advance in the past 10 days will not deal definitively with the jihadis, and that they will have to be confronted in Syria to be fully defeated.

No consensus yet exists as to what that will require the U.S. to do. Deliberations within the administration are said to be ongoing, the result of both an attempt to build an international coalition and a deep wariness of becoming mired in an open-ended conflict.

The Pentagon has yet to decide on expanding the U.S. air war into Syria to attack Isis, let alone how a campaign there would develop, officials said Friday. "I'm not going to get ahead of planning that hasn't been done or decisions that haven't been made," rear admiral John Kirby, the Pentagon press secretary, told reporters.

Within the Pentagon, senior officials are torn between viewing a cross-border attack on ISIS as the only viable option, and a reluctance to engage in what could rapidly become a new, bloody and expanding commitment to yet another Middle Eastern conflict, the Guardian says.

The favoured option, according to two administration officials, is to press forward with a training mission, led by elite special operations forces, aimed at making non-jihadist Syrians an effective proxy force. But the rebels are outgunned and outnumbered by ISIS and the administration still has not received $500m from Congress for its rebel training plans.

Pentagon officials said they had yet to work out what the training program would actually look like, where it will be hosted, or if air strikes on ISIS targets in Syria will support it.

For all the internal administration focus on propping up moderate Syrian rebels, the U.S. military would not be able to begin training them until October, the earliest that Congressional approval could be obtained for the required funding and authorisation. Kirby said he was unaware of any "plan to accelerate it".

Nor have critical details for the training program been worked out, despite it being effectively the lynchpin of what the administration considers a long-term plan to defeat ISIS. "I can't tell you where it would take place, or how many people would be trained, and there's still a vetting process that needs to be fully developed here," Kirby conceded, according to the Guardian.