US airstrikes ISIS
A fighter from the Islamic State group holds a piece of what IS says is a US drone that crashed into a communications tower in RaqqaAFP

A pro-Isis Iraqi cell that was busted by Swiss authorities in March was planning a terrorist attack with explosives and toxic gas, according to local reports.

Swiss intelligence was alerted by a western counterpart that three Iraqis were believed to have formed a jihadist militia in their adopted country, according to daily newspaper Tages Anzeiger.

The group allegedly helped around 40 Swiss or Swiss-resident jihadists to travel to the Middle East and join Isis. They also reportedly aimed to acquire explosives and toxic gas for an attack, although it is not clear at which stage of the plot they were in.

They were aged between 28 and 33 and at least one was an asylum seeker. Two of them lived in the regions of Aargau and Schaffhausen, at the border with Germany. The third was a resident of the Syrian capital Damascus.

The development came as Britain edges closer to joining the US-led airstrikes in Syria and Iraq against the Isis (which has renamed itself the Islamic State).

UK Prime Minister David Cameron is expected to hold bilateral talks with his Iraqi counterpart Haider Abadi, who is likely to place a formal request for military action.

"The prime minister will be holding talks at the United Nations in New York over the next two days on what more the UK and others can do to contribute to international efforts to tackle the threat we all face from [Isis]," Cameron's office said in a statement.

"The UK is already offering significant military support, including supplying arms to the Kurds as well as surveillance operations by a squadron of Tornadoes and other RAF aircraft."

Meanwhile, the Pentagon has warned that Isis is already adapting to the raid by using the civilian population as human shields.