A suicide car bomber killed nine US soldiers and injured more than 20 in the single deadliest attack on American ground forces in Iraq for more than a year yesterday. The attack happened north of Baghdad at a base in Diyala province, which borders Iran and has been the scene of fierce fighting involving US and Iraqi troops and Sunni and Shia forces. The casualties, along with the death of another US soldier in a roadside bombing in Muqdadiya, also north of the capital, took the number of US troops killed in Iraq this month to 85. Commanders fear that a security crackdown inside Baghdad, which began in late February, has made areas around the Iraqi capital more dangerous. The US has also sent extra troops to Diyali, where Sunni insurgents and al-Qaida militants are believed to have regrouped following the Baghdad "surge" in American soldier numbers. Most attacks against US forces have involved ambushes, roadside bombs or mortars, although direct attacks on bases are becoming more common. In February, two US troops died at abase in Tarmiya when a suicide car bombing was followed up with a gun attack. Yesterday's blast was the worst such incident for the US since 10 marines were killed by a roadside bomb while on a foot patrol near Falluja in December 2005. In January this year, 12 US troops died when a Black Hawk helicopter crashed in Diyala. An investigation to determine whether or not the aircraft was shot down is continuing. Also yesterday, a British soldier from the 2nd Battalion the Duke of Lancashire's Regiment was killed by gunfire in central Basra - the 145th member of UK forces to have died during operations in Iraq. At least 48 Iraqis died in seven blasts around the country yesterday. Today, two car bombs exploded near the Iranian embassy in Baghdad, injuring six civilians.