The vastly different and separate campaigns to drive ISIS out of Raqqa and Mosul will possibly take another six months, said the top U.S. commander in the region.

"Within the next six months, I think we'll see both conclude," Army Lt. Gen. Stephen Townsend, commander of Combined Joint Task Force-Operation Inherent Resolve, said of the campaigns to take Mosul in northwestern Iraq and Raqqa in northeastern Syria.

On a visit to the U.S. training base at Camp Taji north of Baghdad, Townsend said he expects the operation to liberate western Mosul -- led by Iraqi Security Forces and backed by the U.S.-led coalition -- to begin "in the next few days," The Associated Press reported.

He did not give a timeline for the start of an assault on Raqqa, the self-proclaimed capital of the Islamic State of Iraq and Syria, where a mix of Syrian Arab and Syrian Kurdish militias, supported by the U.S., is in the "isolation phase" of the effort to take the city, according to Air Force Col. John Dorrian, a task force spokesman.

The campaign against Mosul, which is split by the Tigris River, began Oct. 17 and succeeded in taking the eastern sector in January after street fighting described by the U.S. as "three dimensional," as ISIS defenders fought the ISF's elite Counter Terror Services units from rooftops, alleyways and a maze of tunnels beneath the streets.

In a video briefing to the Pentagon, Dorrian said ISIS is still firing mortars from the western side of the Tigris into the eastern sector and sending drones to drop small munitions, but the attacks are having "limited effect" and will not delay planned operations to take the more densely populated and built-up western side of the city.

The campaign to retake Raqqa is complicated by the Russian military presence backing the regime of Syrian President Bashar al-Assad, Turkey's cooperation with Russia, and the more than five-year-old Syrian civil war that has spawned ethnic and political rivalries across the region.

The U.S. deals differently with the groups making up the Syrian Democratic Forces, which are pressing from the north on Raqqa. Dorrian has repeatedly said that the U.S. arms the Syrian Arab Coalition within the SDF, but does not arm the Syrian Kurdish People's Protection Units, or YPG, which has been branded a terrorist organization by NATO ally Turkey.

Turkey late last year joined with Russia and Iran for talks on arranging a ceasefire in Syria following the fall of Aleppo to Assad's forces, backed by Russian airpower. The U.S. was excluded from the ceasefire talks in Moscow.

Dorrian said the advance on Raqqa is proceeding. "What we would expect is that within the next few weeks, the city will be nearly completely isolated, and then there will be a decision point" on which forces will actually undertake the assault into the city, he said.

Turkey is adamantly opposed to having the YPG, by far the most effective rebel fighting force in Syria, enter the city. Turkish officials have suggested that their military might join the assault.

The split between the U.S. and Turkey on how to proceed against ISIS in Syria led the U.S. to withhold air support for Turkish forces clearing border areas, but Dorrian said the U.S. is now conducting airstrikes coordinated with the Turkish military in the effort to retake the northern Syrian town of al-Bab from ISIS.

On Tuesday, President Donald Trump spoke by phone with Turkish President Recep Tayyip Erdogan on shoring up relations. A White House readout of the call said that Trump "reiterated U.S. support to Turkey as a strategic partner and NATO ally, and welcomed Turkey's contributions to the counter-ISIS campaign."

In Ankara on Wednesday, presidential spokesman Ibrahim Kalin said Erdogan had renewed Turkey's demands for the creation of a "safe zone" inside Syria for refugees and for the extradition of exiled Muslim cleric Fethullah Gulen, Turkey's Anadolu news agency reported.

Gulen, a former Erdogan ally who is living in Pennsylvania, has been blamed by Erdogan for fomenting the failed military coup in Turkey last July.

In an ABC-TV interview last month, Trump said, "I'll absolutely do safe zones in Syria."

The administration of former President Barack Obama rejected the safe zone concept, saying it would cost too much and require a major commitment of U.S. ground troops to protect the zones.

In a sign of improving relations with Turkey, new CIA Director Mike Pompeo is to make his first overseas trip Thursday for talks with his counterparts in Turkey.

In prepared remarks for a Senate Foreign Relations Committee hearing Thursday, James Jeffrey, the former U.S. ambassador to Iraq and Turkey, said the plan to retake Raqqa "should be done in conjunction with, rather than in opposition to, Turkey."

Jeffrey, a distinguished fellow at The Washington Institute for Near East Policy, said Turkey might be more willing to accept U.S. support of the YPG following the Turkish constitutional referendum in April.

He also said, "if the United States desires a rapid victory over ISIS, it probably will have to commit more supporting forces, and possibly limited ground combat formations."

Last month, Trump gave new Defense Secretary Jim Mattis 30 days to come up with a new plan for the "accelerated" defeat of ISIS. On Wednesday, Mattis spoke by phone with new Iraqi Defense Minister Arfan al-Hayali to discuss the overall ISIS campaign and "planning for operations to liberate western Mosul," the Pentagon said.

Just before stepping down as defense secretary last month, Ashton Carter said that more U.S. ground troops is not the answer in Iraq and Syria, where the U.S. has relied on local partners to carry the fight.

More U.S. troops would mean "fighting on the enemy's terms, which is infantry fighting in towns in a foreign country," Carter told The Associated Press.

-- Richard Sisk can be reached at Richard.Sisk@Military.com.