Military operations aimed at retaking the Iraqi city of Tikrit from the Islamic State jihadist group have caused around 28,000 people to flee their homes, the United Nations said on Thursday.

"Military operations in and around Tikrit have precipitated displacement of an estimated 28,000 people to Samarra," a U.N. statement said.

The United States on Wednesday warned an offensive to retake control of the Iraqi city of Tikrit from Islamic State militants must not fuel sectarian tensions.

"It is important -- and (Iraqi) Prime Minister (Haider al-)Abadi himself has indicated... that this operation should not be used as an excuse or as cover for individuals taking sectarian-motivated retribution," said White House spokesman Josh Earnest. 

The Islamic State group said Tuesday a U.S. citizen carried out a suicide attack against Iraqi forces involved in an operation to retake the jihadist-held city of Tikrit.

"The brother Abu Dawud al-Amriki (may Allah accept him) launched himself with his explosives-laden truck against a gathering" of Iraqi forces and militiamen, an IS radio bulletin said.

A suicide car bomb killed nine Shiite militiamen in an attack near the Iraqi city of Tikrit on Wednesday, army and hospital sources said.

The suicide attacker rammed his explosives-laden vehicle into a gathering of Shiite fighters around 20 kilometers (12 miles) south of Tikrit.

India said Friday it was still searching for 39 construction workers abducted in Iraq in June, denying a report claiming they had been executed by the Islamic State group.

Foreign Minister Sushma Swaraj urged the workers' families not to lose hope, despite not knowing the whereabouts of the men who were rounded up by jihadists in Mosul city.

The Iraqi army's next target is to isolate militants holding the city of Tikrit, after having broken the Islamic State group's hold on Baiji and its refinery, a senior officer said Monday.

"Over the next three days, we will work on reinforcing our capacity because we are planning to clear areas on three sides of Tikrit," an army brigadier general said.

Iraqi forces recaptured the strategic oil town of Baiji Friday in a significant victory over the Islamic State group, as the U.N. accused the jihadists of crimes against humanity in neighboring Syria.

Baiji is the largest town to be retaken by government troops since IS-led militants overran much of Iraq's Sunni Arab heartland in June in their bid to create an Islamic "caliphate".

Islamic State militants executed an Iraqi news cameraman, his brother and two other civilians in public Friday in a village north of Baghdad, the journalist's relatives said.

The jihadists shot dead Raad al-Azzawi, a 37-year-old cameraman for local news channel Sama Salaheddin, and the other three victims in Samra, east of the city of Tikrit.

The jihadists controlling the Iraqi city of Tikrit have blown up a Muslim shrine and rigged one of the region's oldest churches with explosives, security sources and witnesses said Thursday.

A police lieutenant from the area around the city, which is controlled by the Islamic State group (IS), said Al-Arbain mosque was dynamited on Wednesday.

Iraqi forces battled Sunni militants along a string of fronts on Tuesday, including at Saddam Hussein's hometown Tikrit, as the United Nations readied a massive aid operation for displaced Iraqis.

Kurdish and federal forces, who wrested back control of Iraq's largest dam, fought jihadists in the country's north, buoyed by intensifying U.S. air strikes and Western arms deliveries.