The U.S. government is offering rewards totalling $20mln for information on what it says are four leaders of the Islamic State (IS) militant group.

It named the four as Abd al-Rahman Mustafa al-Qaduli; Abu Mohammed al-Adnani; Tarkhan Tayumurazovich Batirashvili and Tariq Bin-al-Tahar Bin al Falih al-'Awni al-Harzi. They join a list of suspects sought under the Rewards for Justice Program.

On Tuesday, May 5, IS said it was behind an attack in Texas. It said "two soldiers of the caliphate" had attacked a Prophet Muhammad cartoon contest being held at a conference centre in Garland near Dallas.

The U.S. State Department offered up to $7mln for information on Qaduli whom it described as a senior IS official who originally joined al Qaeda's affiliate in Iraq. It offered $5mln each for Adnani and Batirashvili and up to $3mln for Harzi.

It describes Adnani as an official IS spokesman; Batirashvili as a battlefield commander in northern Syria and Harzi as chief of IS's suicide bombers.

IS has seized swathes of territory in eastern Syria and northern Iraq, declaring them a caliphate and imposing a harsh interpretation of Islamic law on the inhabitants.

The State Department said the group was responsible for systematic human rights abuses, including mass executions, rape and the killing of children.

The highest reward offered under the Rewards for Justice scheme is up to $25mln for Ayman al-Zawahiri who was named leader of al Qaeda in June 2011, shortly after Osama Bin Laden's death.

It also offers up to $10mln for IS leader Abu Bakr al-Baghdadi.

On Tuesday, a report on IS's al-Bayan Radio said the exhibition in Garland had been "portraying negative pictures of the Prophet Muhammad".

The contest had offered a $10,000 prize for a cartoon of the Prophet Muhammad. Depictions of Muhammad are offensive to many Muslims.

Both gunmen were shot dead by a police officer after they opened fire outside the venue on Sunday. U.S. officials later said they doubted the group's direct involvement.