U.S. Treasury Secretary Jack Lew said Tuesday the U.S. is working hard to make sure that the new U.N. sanctions on North Korea are implemented, saying such measures would only be "theory until you implement them."

Speaking at a House Appropriations Committee hearing, Lew said the U.S. has been holding discussions with China on sanctions implementation. Acting Under Secretary for Terrorism and Financial Intelligence Adam Szubin is on a visit to China for such discussions, he said.

He also said China's support of the tough sanctions is a "very significant development."

"I was in China the week after the U.N. Security Council at the G-20 meetings, and I had conversations at the highest levels in China, and I can tell you that they don't view this as something they're doing for us," the secretary told the hearing.

"They look across their border and it makes them very nervous that they can't explain some of the actions that are reckless and that are destabilizing," he said. "They have certainly indicated a high level of concern ... That's why they supported the resolution. They've indicated an intention to implement it."

Implementing the sanctions takes sustained efforts, he said.

"We know from these sanctions programs that it's grueling day-to-day work. You've got to identify the entities, act against the entities," he said. "I think it is a very significant statement to North Korea that China is part of this international effort."

The new U.N. sanctions, the toughest ever to be imposed on Pyongyang, require mandatory inspection of all cargo going in and out of the North and ban its exports of coal, iron and other mineral resources, a key source of hard currency that accounts for nearly half of the country's total exports.

It also prohibits all small arms and other conventional weapons from being sold to the North, bans jet and rocket fuel supplies to the country, grounds North Korean flights suspected of carrying contraband and denies vessels carrying illicit items access to ports.

The Treasury Department is in the final stage of putting together a package of unilateral sanctions against the North to carry out the Security Council sanctions and the recent congressional legislation tightening the screws on Pyongyang. (Yonhap)