NAJAF, Iraq - The Iraqi forces' operation to retake the city of Tikrit has stalled as troops suffer heavy casualties at the hands of Islamic State militants, raising concerns over whether the pro-government fighters are ready for major offensives.

After two days of little activity on the battlefield, Iraq's interior minister, Mohammed al-Ghabban, confirmed Monday that the offensive has "temporarily stopped." The steady flow of caskets arriving in the Shiite holy city of Najaf suggests a reason for the pause; cemetery workers say as many as 60 war dead a day have been arriving.

Since last week, Iraqi forces have hemmed in the Sunni militants in Tikrit, claiming control of the majority of the former Islamic State stronghold. But the operation has come at a cost, with soldiers saying the fight has been tougher than expected. As the momentum has slowed, some Iraqi officials have begun to publicly call for U.S.-led air support.

While Iraqi officials still express confidence that they can retake the city, the stuttering offensive does not bode well for the more complex battles for the city of Mosul or militant-held areas of Anbar that were expected to begin in coming months.

"It's a furious fight, harder than we thought," said Taher Sabah, a 25-year-old militiaman with the Shiite Badr Organization, who arrived to lay his father to rest in Najaf on Sunday after he was killed near Tikrit the night before.

"It's taking so long," he said. "But we'll keep going."

The Iraqi government is not providing casualty figures from the battle. But Najaf's Wadi al-Salaam cemetery, the largest in the world, is the final resting place for the majority of Iraq's Shiites and, given the largely Shiite makeup of the country's security forces, a large number of the pro-government battlefield casualties.

The cemetery's reception office has recorded 127 battlefield deaths since the start of the Tikrit offensive at the beginning of the month. But it does not include numbers for the largest Shiite militias, the Badr Organization, Asaib Ahl al-Haq and Shiite cleric Moqtada al-Sadr's Peace Brigades, who record their figures separately.

At the cemetery's washing stations, where most bodies are taken to be prepared for burial, workers say the numbers are higher.

Kurdish forces in Iraq, meanwhile, are investigating two other possible chemical weapons attacks by Islamic State, a top official said Monday. The two purported chemical weapon assaults resemble one claimed Saturday by Kurdish officials who say an independent laboratory concluded the militants used chlorine gas against its peshmerga forces in a Jan. 23 truck suicide attack. However, their claims were not immediately verified by international authorities.