The United States on Wednesday denounced a deadly suicide attack at a Shiite mosque in northern Iraq, and said it proved "terrorists are willing to target all Iraqis, regardless of their religion."

The attack, the deadliest in six months, is likely to heighten tensions in Iraq which is grappling with more than a month of protests in Sunni-majority areas, amid demands that Shiite Prime Minister Nuri al-Maliki stand down.

In a statement, US State Department spokeswoman Victoria Nuland described the bombing at the mosque in Tuz Khurmatu, 175 kilometers (110 miles) from Baghdad, as "horrific and heinous."

A local official put the toll at 42 dead and 75 wounded.

The aim of the attack was to "expressly undermine the will of the Iraqi people who overwhelmingly support stability and security," said Nuland, adding that "we express our deepest sympathies to the families of the victims."

No group claimed responsibility for the bombing, but Sunni militants often launch attacks in a bid to destabilize the Iraqi government and push it back toward sectarian violence that peaked between 2005 and 2008, in the wake of the US-led war that ousted long-time dictator Saddam Hussein.