Defense Secretary Jim Mattis said Thursday that the United States would regret installing a holding force in Syria without American involvement, indicating military leaders harbor reservations about a White House effort to task Arab militaries with stabilizing areas liberated from the Islamic State. 

Mattis, under questioning by the Senate Armed Services Committee, offered support, too, for lasting U.S. participation, alongside NATO allies, in the military mission in Iraq to maintain security and prevent the Islamic State from reconstituting itself there.

His comments come as the Pentagon faces an inflection point in both countries. Most of the territory once held by the Islamic State has been liberated, but the threat of a resurgence by the Sunni extremist group remains in some areas. 

Mattis’s comments also underscore the tension over how involved the U.S. military should be in controlling future security threats in Iraq and Syria, now that the Islamic State is on its last legs militarily after nearly four years of war.

Military leaders’ open-ended plans for stabilizing Syria were upended by President Trump’s demand for a near-term troop withdrawal. A force of about 2,000 U.S. troops has been working with Kurdish-led militias there to roll back the Islamic State’s self-declared caliphate.

In keeping with his desire to share military burdens with allies, Trump has asked other countries, including Saudi Arabia, to contribute billions of dollars to the stabilization effort in Syria. The White House is exploring the possibility of fielding an Arab fighting force from other countries in the region to prop up cities destroyed by years of bombing and street warfare.

Saudi Arabia has said it would send troops if such a force comes together, but officials have not revealed details of the initiative. A multinational ground force from Arab countries would be highly unusual and would face an array of challenges, including limited experience with external operations by those militaries, political divisions among Arab leaders and risks of stoking broader sectarian conflict.

While Mattis and other military leaders have welcomed the effort to obtain fresh assistance, they have also voiced caution about what such a transition might entail.  

Speaking in a phone interview this week, Gen. Joseph L. Votel, head of U.S. Central Command, said the United States would welcome contributions in forces, equipment or funding from Middle Eastern allies as they seek to stabilize cleared areas of Syria and ensure that the Islamic State cannot return. Votel also suggested that contributions by Arab nations had not been finalized and said that new forces would require time to hone their operations in an environment as complex as Syria’s. 

“We’ve got a very experienced team on the ground that’s been orchestrating this for a couple of years. It would be difficult for someone to immediately step in and replace us, but certainly with time and experience, or maybe a little different situation on the ground, perhaps they could do that,” Votel said Wednesday. 

He said the U.S. military was talking with partners in the region but wants to finish the job it was sent to Syria to do. “So as we add capabilities, we need to ensure that they can be well integrated,” he said. 

Discussion of an Arab force comes as the Trump administration seeks to reconcile what appear to be divergent characterizations by the Pentagon and the White House about what constitutes a lasting defeat of the Islamic State in Syria and how close the United States is to victory.

Speaking at a news conference with French President Emmanuel Macron this week, Trump said he favors leaving Syria and praised American troops for defeating the Islamic State there and in Iraq. 

A day later, Mattis said operations remain unfinished and that the U.S. military is planning to expand its fight against the Islamic State in coming weeks. The effort will concentrate on the Middle Euphrates River valley, an area in Syria that the Pentagon has identified as one of the group’s final bastions, Mattis said, adding that French special forces recently arrived to help. 

The Syrian Democratic Forces, the Kurdish-led militia that American troops are backing in Syria, made progress in the valley this year but were diverted from the fight when Turkish forces launched an incursion against Kurdish positions in the country’s north. Mattis said the U.S. military is about to renew its efforts. 

The chairman of the Joint Chiefs of Staff, Gen. Joseph F. Dunford Jr., warned in testimony Thursday that the hasty withdrawal of U.S. troops from Iraq in late 2011, when the Iraqi government was too weak to secure its territory, gave the Islamic State “space to grow.”

Since launching its offensive against the Islamic State in 2014, the U.S. military has helped local forces in Iraq and Syria recover nearly all of the territory that the extremist group once held. Though the numbers are far from exact, estimates suggest that between 1,000 and 3,000 militants remain in both countries.

The U.S. military has limited its role in Syria to the defeat of Islamic State, apart from two strikes against Syrian leader Bashar al-Assad’s forces to deter their use of chemical weapons. In the meantime, Assad’s forces have consolidated gains in the country, moving toward victory over rebel groups in Syria with the help of Russia, Iran and Lebanese Hezbollah.

A U.S. military departure would be likely to result in greater Iranian influence in Syria, a predicament for an administration that wants to withdraw U.S. troops but also does not want to cede Syrian territory to Iran. 

Trump, in the news conference with Macron, appeared to refer to those concerns. “Emmanuel and myself have discussed the fact that we don’t want to give Iran open season to the Mediterranean, especially since we really control it to a large extent,” Trump said. “We really have controlled it, and we’ve set control on it. So we’ll see what happens.”

Mattis said American troops also are planning greater operations against the Islamic State in Iraq.  

“We’re continuing the fight, and we’re going to expand it and bring in more regional support, is probably the biggest shift we’re making right now,” Mattis said.

Speaking separately, Votel said the United States was ready to continue its military assistance to the Iraqi government, help that has involved advanced military capabilities and support to Iraq’s special forces, if asked to do so after that country’s May parliamentary elections.

The responsiveness the U.S. military has shown to Iraqi requests in the battle against the Islamic State and “our low-visibility aspect” in operating alongside local forces will help secure ongoing U.S. influence with a key partner, Votel said.