US looks at options for action in Iraq as Islamic State ramps up attacks while senior Tories call for UK to join in air strikes

Western powers are coming under mounting pressure to do more to confront Islamic State (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.

As US 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 the Pentagon insisted that it had yet to decide on whether to expand the US air war into Syria.

But Isis has demonstrated its rampant authority in northern Syria in recent days, with the brazen murder of the US 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.

US 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 US 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 US 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.

But in the UK, there were growing calls for a more robust approach, with senior Conservatives including former foreign secretary Sir Malcolm Rifkind and former defence minister Sir Gerald Howarth, calling for the UK to join the US and target Isis in Syria.

"I think we have to wake up and be a bit more grown up in the west about what is in our national interest," Howarth said. He acknowledged that the government is wary of ordering air strikes because it lost a Commons vote last summer on military action against Syria. But he said there should be "a strategic rethink of what is in the British national interest" and that this time MPs could be persuaded to vote for a bombing campaign.

But engagement in Syria itself is fraught with difficulties, not least because it would look like collaboration with the Assad regime. A year ago, after the gas attack in Damascus, those who urged a bombing campaign in Syria wanted Assad to be the target, not the beneficiary. The irony is an uncomfortable one for policymakers.

"I do not think that engaging in a dialogue with the Assad regime would advance the cause that we are all advocating here," said the British foreign secretary, Philip Hammond. "We may very well find that we are aligned against a common enemy. But that does not make us able to trust them."

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 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 US 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.

The broader intention is to try to strip Isis of the support of the 20 million Sunni Arabs who live between Damascus and Baghdad. But the difficulties of that approach was underscored when Shia militia gunned down dozens of Sunnis in a village north of Baghdad, killing at least 68 in one of the deadliest attacks this year.

The White House resisted efforts to portray a rift with the Pentagon, insisting it agreed with Thursday's comments by General Martin Dempsey, chairman of joint chiefs of staff, about the cross-border nature of the threat and claiming it was already responding by helping moderate Syrian rebels.

"We certainly agree that any strategy to deal with Isil has to deal with both sides of the border, Iraq and Syria," said national security adviser Ben Rhodes, using the Obama administration's preferred acronym for Isis. "The strategy that we are already undertaking does address that."

However, the White House went further than before in its condemnation of Isis, describing the killing of Foley as an act of terrorism. "When we see somebody killed in such a horrific way, that represents a terrorist attack against our country and against an American citizen, Rhodes said, saying the US would do whatever necessary to protect Americans in future.

"We are actively considering what is necessary to deal with that threat and we are not going to be restricted by borders," said Rhodes, briefing reporters at Martha's Vineyard, where the president is on vacation.

Additional reporting by Dan Roberts in Washington