Iraqi Prime Minister Haider al-Abadi received an invitation from King Salman to visit Saudi Arabia, as relations between the two nations improve, the premier's office said.

Ties between Baghdad and Riyadh have become steadily better since Abadi took office last year after reaching a low ebb under his predecessor Nuri al-Maliki, who accused Saudi Arabia of backing militants in Iraq while it criticised him as sectarian.

Iraqi President Fuad Masum visited Saudi, and Riyadh announced in January that it would reopen an embassy in Iraq for the first time in more than two decades.

The invitation came during a phone call with the king in which Abadi sought to emphasise the cross-sectarian nature of Baghdad's battle against the Islamic State jihadist group, which led an offensive that overran large parts of Iraq last June.

Victories over IS "were realised with the cooperation of all the sons of the Iraqi people and by volunteers from the sons of the areas," Abadi told Salman, according to a statement released Sunday night.

Abadi also said that there is a strategy to hand over Sunni Arab-majority areas retaken from IS to the local police.

While there are Sunni tribesmen and volunteer fighters battling IS, most of the security forces and allied paramilitaries fighting the jihadists are from Iraq's Shiite majority.

Some Shiite militiamen have been accused of carrying out human rights violations in retaken areas.

Abadi has sought to reassure both Iraq's Sunni minority and neighbouring Sunni-majority countries that his government is neither sectarian nor working against them.