Driving up sanctions on North Korea is expected to be a main pillar of a Hillary Clinton administration policy on the communist nation, as her likely defense secretary said pressure is the only way to force Pyongyang back to serious talks.

Michele Flournoy, who served as an under secretary of defense during President Barack Obama's first term, said in a wide-ranging interview with Yonhap News Agency that there is no point in holding negotiations with the North unless it's prepared to talk about disarming.

"They have to be motivated by sanctions and they have to demonstrate their seriousness in some way, whether it's stopping these provocations and/or taking some of the steps that they've promised in the past to show that they're curtailing their nuclear weapons activities," Flournoy said.

The interview was a chance to preview what Clinton's policy on Korea would look like, as Flournoy is considered a top foreign policy and national security adviser to Clinton. She has also been widely talked about as the top candidate for defense secretary under a Clinton White House.

Flournoy tamped down calls for an early reopening of negotiations with the North.

"Negotiations are a waste of time unless you have those (denuclearization) signals from them," she said.

When it comes to dealing with Pyongyang, Flournoy said that all options are on the table, including pre-emptive military action, adding that pre-emptive action is even in the UN charter when talking about self-defense against imminent threats.

Still, however, the focus should be on strengthening sanctions, she said.

"I think all options would be on the table, but the focus of our policy should be to try to put through sanctions and other pressure, get the leadership to come back to the negotiating table and make good on their earlier promises of denuclearization," Flournoy said.

She stressed that sanctions on Pyongyang should include cutting off energy supplies to the North and restricting trade along the long North Korean-Chinese border. She said the US should engage China seriously to have it enforce sanctions to rein in the recalcitrant neighbor.

"We know that there is illicit activity going on right now that is already in violation of existing sanctions and much of that is coming through China," she said. "We need to engage China to get them to try to be more serious about enforcing existing sanctions, let alone future sanctions on their own soil."

Last month, the US imposed sanctions on a Chinese company for providing the North with "dual-use" items that can be used in nuclear and missile development. It was the first time the US has ever sanctioned a Chinese firm with regard to Pyongyang's weapons programs.

Flournoy said that such sanctions will continue.

"We are trying to ensure that the sanctions are actually implemented and enforced. I'm sure the US will use its diplomacy anywhere where those sanctions are being violated," she said.

Flournoy also said that the US and South Korea, while ramping up sanctions, should also make sure to have the "full range of options for deterrence."

She also said the US commitment to South Korea's security shouldn't be questioned.

"The US needs to work hard to reassure South Korea that their security is guaranteed by the US, all means at our disposal up to and including our nuclear deterrent if necessary," Flournoy said.

That differs sharply from accusations by Republican candidate Donald Trump that allies are free-riders sucking up American taxpayer dollars for their own defense and that the US should be prepared to abandon them unless they pay more.

Flournoy stressed just how important it is to have allies and to honor security commitments to them. Clinton has also said repeatedly during her campaign that alliances are a "source of strength" that makes the US safer.

"I think most Americans, most members of Congress, most presidential candidates understand that in a very complex dynamic security environment we gain a great deal from having strong allies with shared interests and shared values who are willing to work alongside us to create stability and security," Flournoy said.

Flournoy, currently CEO of the Center for a New American Security, a major think tank in Washington, is scheduled to visit South Korea early next week with a group of next-generation leaders from the think tank.

While in Seoul she is expected to hold a series of meetings with senior South Korean officials. (Yonhap)