BAGHDAD (Reuters) – U.S. Defense Secretary Jim Mattis arrived in Iraq on Tuesday, just days after the start of an offensive to take back the city of Tal Afar, to speak with Iraqi leaders about the next steps in the fight against Islamic State.

Mattis has warned that the end of the militant group is far from close. Iraqi security forces launched an offensive to take back the city of Tal Afar on Sunday, their latest objective in the U.S.-backed campaign to defeat Islamic State militants, Prime Minister Haider al-Abadi said.

A long-time stronghold of hardline Sunni Muslim insurgents, Tal Afar, 80 km (50 miles) west of Mosul in Iraq’s far north, has experienced cycles of sectarian violence between Sunnis and Shi’ite Muslims.

“ISIS’ days are certainly numbered, but it is not over yet and it is not going to be over anytime soon,” Mattis told reporters in Amman.

Mattis said that after retaking Tal Afar, Iraqi forces would move against the western Euphrates river valley. He added that Iraqi security forces were capable of carrying out simultaneous operations.

Brett McGurk, the U.S. special envoy to the coalition against Islamic State, told reporters that while the battle for Tal Afar would be difficult, Iraqi forces had retaken 235 square kilometers (90.7 miles) in the first 24 hours.

U.S. officials, speaking on the condition of anonymity, said Mattis, who will meet Abadi and Defense Minister Arfan al-Hayali, would discuss the future of U.S. forces in Iraq after the fall of the remaining cities under Islamic State and the role they could play in stabilizing operations.

The officials said that while major cities like Mosul have been largely been cleared of Islamic State militants, there were concerns about the ability of Iraqi forces to hold territory.

Mattis said pockets of resistance remained in west Mosul, including sleeper cells, and attention would be turned after they were cleared.

“It is not going to happen overnight…it is going to be a heavy lift for them going forward, but the proper governance would involve local representation in their day to day lives,” Mattis said.

Islamic State is also on the back foot in Syria, where Kurdish and Arab militias backed by a U.S.-led coalition have captured swathes of its territory in the north and are assaulting its former Syrian “capital” of Raqqa.

McGurk said that about 2,000 Islamic State fighters remained in Raqqa and as much as 60 percent of the city had been retaken.

The jihadist group is now falling back deeper into the Euphrates valley region of eastern Syria.

Mattis added that the next step for forces fighting Islamic State in Syria would be to move against the middle Euphrates valley, in a reference to the militants’ stronghold in Deir al-Zor province southeast of Raqqa.

KURDISH REFERENDUM

Iraq’s Kurds have said they will have a referendum on independence on Sept. 25 despite concerns from Iraq’s neighbors who have Kurdish minorities within their borders, and a U.S. request to postpone it.

However, a senior Kurdish official said Iraq’s Kurds may consider the possibility of postponing the referendum in return for financial and political concessions from the central government in Baghdad.

McGurk said the Kurdish delegation’s recent visit to Baghdad was encouraging.

A U.S. official said Mattis would press Massoud Barzani, president of the autonomous Kurdistan Regional Government (KRG), to call off the referendum.

The Pentagon signed an agreement with Peshmerga forces last year to provide hundreds of millions of dollars in weapons and equipment.

The U.S. official, speaking on the condition of anonymity, said that the memorandum of understanding would expire soon and suggested that Mattis could use it as a bargaining chip.

The United States and other Western nations fear the vote could ignite a fresh conflict with Baghdad and possibly neighboring countries, diverting attention from the ongoing war against Islamic State militants in Iraq and Syria.