Sanctions are not a "short-term game" but a longer-term one that needs time and cooperation with other major countries to make them successful, the top U.S. diplomat in Seoul said Wednesday.

China is an important part in all of this, and that is why it is necessary to keep engaging with Beijing at "very senior levels," Mark Lippert, U.S. ambassador to South Korea, told a forum in Seoul.

"I think the important thing to remember is that sanctions aren't just a short-term game. It is the longer-term game," he said. "That's really one of the key lessons we, the United States, took away from the Iran context."

The remarks were made at a forum held in Seoul in response to questions about the latest failed missile test by the North and toughened sanctions slapped on Pyongyang after its fourth nuclear test and other provocations earlier this year.

The U.S. led the sanctions, followed by enforcement of their own by some major countries, including Switzerland and Russia. The European Union announced its own toughened sanctions on the North last year.

Lippert emphasized that a "multilateral" approach is important in getting the North to give up its nuclear weapons.

"The other big lesson that we took away from the Iran context was ... you have to have multilateral sanctions that are working together with the Europeans, with Chinese, with the Russians," the ambassador said.

Lippert said that there have been progress and signs that China, in particular, has been taking some actions on the North.

"We have some interesting things where Chinese instructed some of their ministries and customs agencies to undertake more robust enforcement. Again, it's early and the jury is still out. That's precisely why the U.S. and the Republic of Korea continue to follow up with the Chinese."

"This is a medium- and long-term endeavor. We need to keep engaging the Chinese at very senior levels to make progress on this really important issue," he added.

The ambassador noted that the North Korea issue will be a key agenda item at the strategic economic meeting between the United States and China to be held in a few weeks. (Yonhap)