Defense Secretary Jim Mattis told Congress Wednesday that the Pentagon is still working on its revised strategy to defeat the Islamic State in Iraq and Syria, and it may be weeks or months before it is finished.

In testimony before the Senate Appropriations subcommittee on defense, Mattis said, "We've got the skeleton plan put together. We're fleshing it out."

Mattis described the strategy as an "interagency-developed report" that embraces "economic, diplomatic, military, covert means."

"We should have this done in the next couple of months, if that long," Mattis said. "It may not even take us another month. But we're still putting it together."

Testifying alongside Mattis, Joint Chiefs Chairman Gen. Joseph Dunford described the effort to liberate Raqqa, the Islamic State's self-proclaimed capital in Syria, as complicated by the shifting loyalties of some of the indigenous fighters backed by the U.S., including elements of the Kurdish faction known as the YPG.

"I'm going to be quite honest with you. The group that we are supporting, certainly at the political level, has been engaged in Russia," Dunford testified. "The YPG has a political office in Moscow itself, but the groups that we're providing support to on the ground are not being supported directly by Russian military forces."

Mattis said the faction of YPG fighters who are aligned with Russia are "Afrin Kurds," that is Kurds who come from the Afrin area of Syria.

"I can confirm for you that the specific group that's being supported by the Russians is not a group that has received training, equipment, resources from us in the northwest part of Syria," Dunford said.

"In the specific groups that we do provide support to and the ones that we have asked to provide additional support to, we do have a very detailed vetting process that we use to mitigate the risk of that weapons or equipment falling in the wrong hands," he said.