The two biggest prizes in the war against the Islamic State in Iraq and Syria are its de facto capitals, Mosul and Raqqa.

And the top U.S. general in charge of that region said Tuesday that Iraq is on course to meet its goal of liberating Mosul before the end of the year.

"My assessment is in over the course of my visits I think is that they are on track to achieve that objective," U.S. Central Command chief Army Gen. Joseph Votel told reporters at the Pentagon Tuesday. "I think we are proceeding apace exactly where we hope to be at this particular time."

But Votel also warned that after the inevitable defeat of the Islamic State, also called ISIL, in its two strongholds, the U.S. will face what he called "next evolution of ISIL" in which the group is likely to revert to terror tactics, rather than defend territory.

Current ISIS Territory Status in Iraq and Syria Graphiq

"We should expect that as we come out of the big operations like Mosul and Raqqa and others here, that they will continue to adapt," Votel said. "We will continue to deal with the next evolution of ISIL, whether they become more of a terrorist organization and return to more of their terrorist-like roots."

Votel said that while the Islamic State has proved it is vulnerable, it has also shown it's adaptable and can pose a threat outside its self-declared caliphate.

"As we've learned about this enemy, one they are very connected. So things that happen in Iraq and Syria resonate in other places and they resonate in our capitals in Europe and other locations, so it's a connected network," Votel said.

Overall Votel delivered an optimistic assessment of progress on the battlefield, portraying the Islamic State as under "more pressure than any other time in the campaign," having to "look in multiple directions," and "struggling to respond."

"ISIL is having to make hard decisions because they're getting pressured in a variety of ways," Votel said. "We are continuing to target their leadership or continuing to target their revenue generation sources in both Mosul and northern Iraq and certainly in Syria, so I think we continue to keep them on the horns of a dilemma here."

Votel praised Turkey's incursion into northern Syria this week to aid Syrian fighters who cleared the border town of Jarabulus of Islamic State forces, calling Turkey's operations along the border "extraordinarily important and welcome."

But Votel admitted the Turks gave the U.S. little notice, and that attacks over the weekend against U.S.-backed Kurds, which Ankara considers terrorists, were counterproductive.

"What we have made clear is that our support to all parties is contingent upon the focus on ISIL," Votel said. "It's not helpful to have infighting among themselves. We don't want that. We're working to prevent that."

Votel said Syrian Kurds have now largely moved out of the area along the border where Turkish forces are operating, and Turkish forces have not been moving west along the border avoiding areas to the south where there are more Syrian fighters that include Kurdish elements.