SPIEGEL: Ambassador McGurk, in recent days we have been hearing announcements of a strike on Raqqa in Syria, an Islamic State (IS) stronghold and an advance towards Fallujah in Iraq. Is a major assault against IS now starting?

McGurk: We are moving into a new phase of the campaign against Islamic State. Our goal is to attack the enemy and put simultaneous pressure on multiple points. The major offensives are starting on Fallujah and Raqqa to shrink the territory of the self-proclaimed caliphate of the Islamic State and to show that they are not expanding anymore.

SPIEGEL: There has been talk of the offensives for quite a while. Why are they only just beginning now?

McGurk: We needed some time. It took a while time to build military capacity on the ground, to recruit local forces, fighters from the region, to train and advise them. Now we are at a point where they can take on the fight.

SPIEGEL: How strong is Islamic State today?

McGurk: They are substantially degraded. Forty-five percent of the territory in Iraq has been lost, 20 percent in Syria. We are taking out one of their battlefield commanders almost every three days because we are inside their networks. We know where they are and we can target them with more and more precision. After some intensive work by the intelligence services, we are also targeting their financial system. We found out where they are storing their cash. After some work by the US special forces on the ground, we know where they get their oil out of the ground, how they transport it and we hit those sites.

SPIEGEL: Will airstrikes be sufficient for fighting them?

McGurk: Don't take my word on that, listen to them. Back in 2014, their leader Abu Bakr al-Baghdadi announced his caliphate was expanding all over the Middle East and into Europe. Now the key spokesman puts out a very different message, admitting that their territory is shrinking, that their leaders are being killed one by one, even that Raqqa and Mosul will fall. It's a very different message. They are no longer able to claim they are a divine movement; they are a criminal terrorist organization. We have exposed their lies by beginning to beat them on the battlefield.

SPIEGEL: When you mention "we," are you referring to the US special forces units?

McGurk: These brave Americans play an extremely important role. The special forces started with 50 men in northern Syria, they started to find local fighters who were willing to take up the fight against the Islamic State, they trained then them and advised them on how to conduct their operations. They took out Islamic State's finance chief, they killed their military war emir. With the special forces, we have increased the pressure and we won't let up.

SPIEGEL: The mission was difficult since Washington said for a long that the US won't put troops on the ground in Syria.

McGurk: I remember a day in May 2015 when I met with President Obama. We were together in the Situation Room in the White House. Islamic State had just taken Ramadi near Baghdad and we were assessing our possible actions. Finally, the president authorized the opening of a small special forces base near Ramadi. By being there we had the chance to reach out to the local tribal leaders and help the Iraqi forces to plan a counterattack push on the city to take Ramadi back. It was a success. We learned from that incident that where we have presence, even in very small numbers, where we gather local knowledge, where we build relationships by reaching out to local leaders, we can have success.

SPIEGEL: So does that mean that more units will soon be sent to Syria?

McGurk: It's always a possibility to send in more forces. I think President Obama has been very clear in saying that we only invest in things which are working. If something is working, we will consider if more investment might deliver a quicker result. We want to accelerate and do as much as we can as fast as we can. The number of 300 (special forces soldiers) gives us a great deal of capacity. We'll see where it goes.

SPIEGEL: The Kurds in northern Iraq and Syria are still considered to be a reliable partner for the United States. But the large Kurdish groups -- YPG and the Peshmerga -- don't want to liberate Mosul or Raqqa, Arab-dominated cities.

McGurk: The Kurds will be the first to tell you that they don't want to move down to Raqqa. That's why we work hard at recruiting inclusive local forces which are ethnically mixed. In northern Syria we have built such a force. Of 6,000 fighters, a good 2,500 are non-Kurdish. That is the way to go forward.

SPIEGEL: A SPIEGEL reporter recently traveled near Mosul. He reported that the possible local partners -- Kurds, Shiites and Sunnis -- are already fighting over the future distribution of power in the city.

McGurk: We are not at the point where we can start a push towards Mosul. As in the Anbar province, we want to recruit a force of 15,000 fighters. On the political side we try to have the Kurds and the central government talk more with each other. We need to develop an overall comprehensive plan which everyone agrees on before the last phase begins.

SPIEGEL: What is the plan in the meantime?

McGurk: We're trying to cut off Mosul from all sides -- that's pretty much done, it's almost isolated. Now we are targeting their leaders. We are destroying their cash storage sites so they can't pay their fighters anymore. As we squeeze and isolate Mosul, we target sites with airstrikes and we work with forces inside Mosul who fight Islamic State. We hope it won't be a street-to-street fight. In the end, we want to degrade Islamic State so far that it implodes from inside.

SPIEGEL: During his recent visit to Germany, President Obama asked for greater military support from NATO. What could the alliance do to strengthen the coalition?

McGurk: NATO has a very important role to play. The alliance has started a defense building initiative with Iraqi officers in Jordan and we hope this will expand. NATO's AWACS air platforms can play an important role and they would be a great help for the coalition. I hope that by the time of the Warsaw NATO summit in early July, we will have some concrete objectives for that new mission.

SPIEGEL: Germany is so far only training and equipping local Kurdish forces around Erbil. Is that sufficient?

McGurk: Germany has filled some gaps at a very critical time. Chancellor Angela Merkel equipped the Kurdish forces quickly with anti-tank weapons. These MILAN rockets were crucial in helping the Peshmerga to fend off vehicle-borne improvised explosive devices (VBIEDs).

SPIEGEL: In recent days, Moscow has spoken of the possibility of joint airstrikes with the US. Is that a realistic option?

McGurk: We don't cooperate with Russia on a military level. We have a daily phone call to deconflict the airspace to make sure there are no accidents with our fighter jets. That's it. Russia should instead use its influence on the Syrian regime to make sure it adheres to the Vienna process and ceases the hostilities. We are talking with the Russians about that, but not about joint operations.