White House National Security Advisor Robert O'Brien said on Wednesday that sanctions are a powerful tool but they may take a long time to have their intended effect.

The national security adviser made the remarks in a seminar hosted by the William S. Boyd School of Law at the University of Nevada, Las Vegas, while mentioning the effects of sanctions on Iran, North Korea and Venezuela.

O'Brien said the U.S. has many tools for sanctions, citing the U.S. dollar's status as the world's key currency and secondary sanctions, but it could take years for them to have the intended effect. 

While mentioning sanctions on Venezuela and North Korea, O'Brien said there have been "very hard sanctions" on the North. 

He added that if the countries are willing to kill their own people or have enough money and resources to withstand sanctions, it may take even longer to make an impact on the countries.

The security adviser said Iran, North Korea, Venezuela and extremist terrorist organizations remain as challenges to U.S. security.