Bombing Isis oil infrastructure will not be enough to cripple the financing of the jihadi war machine, western intelligence assessments indicate, as the scale and intricacy of the economy it runs becomes clear.

The US-led coalition against Isis last week celebrated having killed the head of its financial affairs, Abu Salah. President Barack Obama hailed the killing on Monday as part of a concerted effort to strike the jihadi group and its leadership “harder than ever”.

But in a Financial Times investigation into Isis finances, several senior intelligence officials and diplomats have said that without a ground campaign it will be hard to stop Isis from financing its core operations — and paying the more than 30,000 fighters who make up the backbone of its military force.