(CNN) The director of the CIA said Wednesday despite the government's best efforts, the likelihood of terrorist activity in the United States is strong.

"So I think we have to assume there's something here in the states," said John Brennan, in an interview for CNN's "Erin Burnett OutFront" that aired Wednesday night. "We have to be relentless in terms of going after them."

Brennan, who was appointed to lead the CIA shortly before President Barack Obama's second term, said "it's impossible to say" whether ISIS has operatives or cells in the United States, and he credited the "tremendous advances in information sharing and interaction between federal officials" in making it difficult for terrorists to operate in the country.

He said he is confident that the US will be "able to remove other senior members" of ISIS, including the organization's leader Abu Bakr al-Baghdadi.

"His time is limited," Brennan said of al-Baghdadi. "It's just a question of whether or not he is going to be removed this week, this month, next month or in the coming months."

But still, Brennan said "you cannot assume there's nobody in the homeland."

"What you need to do is to be able to continue to uncover and use intelligence, what they might be doing here," he said.

Brennan pointed to the example of Ahmad Rahami, the suspect in the recent bombings in New Jersey and Manhattan, as representative of "the new wave of terrorism that we're going to have to deal with."

"This is really quite challenging for local law enforcement ... individuals can concoct these things in their own basement without anybody else's knowledge," he said.

Brennan said that Rahami, who was arrested last week and faces multiple charges for the powerful blasts that left several with non-fatal injuries, carried out the bombings with a "relatively limited amount of support."

Looking beyond US shores, Brennan said Russia is "an adversary in a number of areas," and he noted Moscow's aggression in cybersecurity. Intelligence officials have indicated that Russia was behind the recent hack of the Democratic National Committee, but Russian President Vladimir Putin denied his government's involvement.

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"I will say though that when I look at Russia right now, we know that Mr. Putin has been very aggressive the foreign policy front," Brennan said. "But also very aggressive in the cyber realm...So I think while we have to be very wary of what the Russians may be doing, I think we have to be very careful about believing some of the things that they say publicly or disavowing any types of activities that they're gonna be engaged in."

Turning to North Korea, Brennan called for "strengthening the sanctions" against the hermetic nation to thwart its leader, Kim Jong Un, from developing nuclear capabilities. Brennan called Kim a "megalomaniac" and "calculating."

"I wouldn't say he's reckless at this point because I think he's taken steps that really go up to the brink, but do not go past it, that would generate some type of action against him from either regional players or the United States," Brennan said. "He is determined in terms of having North Korea acknowledged as a nuclear state."

Brennan also said the North Korean weapons program will be a major issue in the transition to a new administration. "That's something that the new team and the current team is looking at very, very closely and will need to be able to address."

An administration official tells CNN that the director believes North Korea is "a front burner" issue and may be the "most likely to cause a crisis quickly" when a new administration takes over, because of the regime's increasing ability to launch mobile missiles and conduct underground nuclear tests with no notice. There is no indication the North is going to launch an attack, but Brennan's view that a new President must be prepared to deal with North Korea within hours of taking office should it engage concerning activities such as further nuclear tests, or move their troops or weapons. That view is shared by other top officials, but Brennan has been the most public to date.

"A new President could have to make decisions very soon," the official said. The US assessment is the Kim Jong Un will use a new US administration to "push for attention" and try to make the world accept as a fait accompli that North Korea is a nuclear state, the official said.

The official points out it is the ability of North Korea to conduct activities with no advance warning and the limited US intelligence about the regime that puts it in a different category than other potential crises during a transition. The US will be watching closely for any cyber or terrorism threats, as it always does. And there is good intelligence on Russian activities. But it's the ability of North Korea to surprise a new President that is the worry.