WASHINGTON (Reuters) - A top U.S. general warned on Tuesday that Islamic State would pose an enduring threat following a planned U.S. withdrawal from Syria, saying the militant group retained leaders, fighters, facilitators and resources that will fuel a menacing insurgency.

The remarks by U.S. General Joseph Votel, head of the U.S. military’s Central Command, represent the latest warning by current and former U.S. officials about the risk of a resurgence by Islamic State following a planned U.S. withdrawal from Syria ordered in December by President Donald Trump.

“We do have to keep pressure on this network. ... They have the ability of coming back together if we don’t,” Votel, who oversees troops in the Middle East as well as Afghanistan, told a Senate Armed Services Committee hearing. He added that territory under Islamic State’s control had been reduced to less than 20 square miles (52 square km) and would be recaptured by U.S.-backed forces prior to the U.S. withdrawal, which he said would be carried out in a “deliberate and coordinated manner.”

Votel told the Senate hearing he was not consulted ahead of Trump’s surprise decision to withdraw America’s more than 2,000 troops from Syria, which helped trigger the resignation of his defense secretary, Jim Mattis.

Trump’s Syria withdrawal has also fueled rare, vocal opposition from within his own Republican party.

The Republican-led U.S. Senate on Monday backed largely symbolic legislation that broke with Trump by opposing plans for any abrupt withdrawal of troops from Syria and Afghanistan.

It warned that “a precipitous withdrawal” could destabilize the region and create a vacuum that could be filled by Iran or Russia.

U.S. officials tell Reuters that the military has already started the withdrawal process, adding hundreds of troops to Syria to facilitate a safe pullout. It has begun to withdraw equipment from Syria and is expected to begin the drawdown of personnel soon.

U.S. Army General Joseph Votel, commander of the U.S. Central Command, testifies before the Senate Armed Services Committee on Capitol Hill in Washington, U.S., March 13, 2018. REUTERS/Aaron P. Bernstein

STATE OF THE UNION

Trump is expected to tout U.S. successes in Syria when he appears before a joint session of Congress to deliver a State of the Union speech at 9 p.m. on Tuesday (0200 GMT Wednesday).

A source close to Trump said the president will declare the militant group all but defeated and will discuss his planned Syria drawdown.

It is unclear whether Trump’s triumphant tone will echo the warnings coming from different parts of his administration, including Votel’s Central Command and the U.S. military.

The Pentagon’s own internal watchdog released a report on Monday saying Islamic State remained an active insurgent group and was regenerating functions and capabilities more quickly in Iraq than in Syria.

“Absent sustained (counterterrorism) pressure, ISIS could likely resurge in Syria within six to 12 months and regain limited territory,” the report from the Pentagon’s inspector general said.

The report, citing information from U.S. Central Command, said Islamic State would portray the withdrawal as a “victory” and conduct attacks on American personnel during the pullout process.

A report by United Nations Secretary-General Antonio Guterres warned that Islamic State has transformed into a covert network, including in its strongholds of Syria and Iraq, but is still a threat with centralized leadership, up to $300 million at its disposal and thousands of fighters.

The 18-page report to the U.N. Security Council, seen by Reuters on Tuesday, described Islamic State as “by far the most ambitious international terrorist group, and the one most likely to conduct a large-scale, complex attack in the near future.”

It said the group was interested in attacking aviation and using chemical, biological, radiological and nuclear materials and that there were up to 18,000 Islamic State militants in Iraq and Syria, including up to 3,000 foreign fighters.

“Foreign terrorist fighters leaving the conflict zone, or prior returnees becoming active again on release from prison or for other reasons, will increase the threat,” the report warned. “Radicalized women and traumatized minors may also pose a serious threat.”