President Donald Trump should stick with imposing sanctions on North Korea and avoid military conflicts, a former undersecretary of state for political affairs told CNBC on Tuesday.

“Right now there are no good military options beyond defense,” said Nicholas Burns, who has served presidents of both parties.

“The Trump administration has to focus on economic issues,” Burns added an interview on “Squawk Box.” “You see the Trump administration talking about a variety of economic options, from cutting off all trade with any country trading with North Korea, which is not something they can seriously consider.”

On Monday, the U.S. ambassador to the United Nations, Nikki Haley, warned the U.N. Security Council that North Korea is “begging for war” after the reclusive state set off its most powerful nuclear bomb yet.

The Trump administration urged its allies to use all possible diplomatic measures and impose urgent economic sanctions to prevent a war.

Burns, who also was U.S. ambassador to NATO, said the Trump administration should begin to impose secondary sanctions.

“To look at companies, Chinese companies, that are cheating on the current international sanctions and make examples of them — that’s probably going to be a more fruitful way to go forward,” said Burns.

Burns, now a Harvard professor, said the Chinese are frustrated with North Korea leader Kim Jong Un and don’t want to see a war.

China, North Korea’s main ally and trading partner, has called for a peaceful resolution to the crisis.

“But the Chinese fear something more,” Burns said. “They fear refugees from North Korea into China.”

—Reuters contributed to this report.