kobani air strike
A US Air Force B1-B bomber plane flies above KobaniAFP

The Pentagon has increased its estimate of the cost of the US air strike campaign against the Islamic State (Isis) to $8.3m a day.

According to Pentagon spokesman Commander Bill Urban, the total cost of the war against the group in Iraq and Syria will amount to approximately $580m.

The US Department of Defense had previously estimated that the average cost of the operation against the radical Islamist militants was $7m per day.

The increase in the Pentagon's estimates is due to the growing number of air strikes and accompanying surveillance flights taking place in Islamic State's self-proclaimed "caliphate", according to a defence official speaking on condition of anonymity.

The operation, which has been named "Operation Inherent Resolve", could cost a total of $2.4bn to £3.8bn a year, according to Todd Harrison of the Center for Strategic and Budgetary Assessments. If the campaign is expanded, the total cost could reach $6.8bn in a single year.

The high cost of such a campaign is due to the large quantity of surveillance and reconnaissance flights that must take place before and after air strikes against terror targets, according to analysts.

The flight of a spy plane can cost $1,000 for an hour flying a Predator drone to $7,000 for a Global Hawk drone.

The Syrian monitoring group, the Syrian Observatory for Human Rights, estimates that at least 800 people - mostly IS militants and Kurdish fighters - have been killed in the month-long assault on the city of Kobani.