WASHINGTON (AFP) — The US Air Force has grounded all its F-15 fighter planes after an accident in Missouri triggered concerns about the ageing fleet, officials said Tuesday.br /br /The decision comes after Japan on Sunday grounded its F-15 fleet after it was informed by US forces that an Air National Guard F-15 fighter jet had crashed.br /br /”The cause of that accident is still under investigation,” the US Air Force said in a statement.br /br /”Preliminary findings indicate that a possible structural failure of the aircraft may have occurred. The suspension of flight operations is a precautionary measure.”br /br /The Air Force has more than 700 F-15s in its fleet. But until further notice, they will only be used for emergency missions in Iraq or Afghanistan, while F-16s will be used for all routine operations.br /br /”They have some concerns about structural issues with the F-15 that went down,” said Pentagon spokesman Bryan Whitman.br /br /”The Air Force is looking at its operational requirements and where they have the newer aircraft and are making some of those available for missions while they take a look at the F-15 and see if they have a more systemic problem throughout the fleet,” said Whitman.br /br /”They will still be flying F-15 in support of troops and when there are no other aircraft available.”br /br /The plane that crashed on Friday was a 27-year-old F-15C built in Hazelwood, Missouri.br /br /Around 500 of the total F-15 fleet are older models with an average age of around 25 years, Air Force spokesman Cristin Marposon told AFP.br /br /There are also around 223 of the later F-15E models, which are mostly the ones used in Iraq.br /br /The Air Force is trying to replace its oldest F-15s with the F-22 Raptor made by Lockheed Martin, but due to budgetary constraints it has so far only been able to purchase half of the aircraft it has asked for.br /br /The US Congress has given approval for the Air Force to purchase 183 F-22s, although military officials have said they need 381 aircraft. A total of 97 F-22s have already been integrated into the fleet.br /br /But Friday’s accident may help step up the replacement program.br /br /”The chief of staff directed the grounding for safety concerns. We will see what comes out of the investigation,” Marposon said.br /br /The US Air Force stopped buying F-15s in 2004, but Boeing has continued to produce them for clients in South Korea and Singapore, which ordered 12 new craft at the end of 2005.br /br /In Japan, Defense Minister Shigeru Ishiba told reporters that Tokyo has also suspended flights of F-2 fighter jets after one crashed on takeoff and burst into flames at an airport in central Japan last week.br /br /”We will deal with the task of preventing airspace incursions with our F-4 fighter jets,” the oldest model among Japan’s fighter jets, Ishiba said.br /br /The Japanese Air Force had 203 F-15s, 68 F-2s and 91 F-4s as of March 2006, a Japanese defence ministry spokesman said.