South Korea's main news agency reported, meanwhile, that the North had rejected an offer for direct talks with the U.S. over its nuclear program.
The U.S. could slow its move toward sanctions if North Korea returns to deadlocked six-nation talks aimed at ending Pyongyang's nuclear program, the official, who is deeply involved in the talks, told The Associated Press.
The Yonhap news agency, citing an unidentified source, reported that the chief U.S. nuclear envoy, Assistant Secretary of State Christopher Hill, proposed a meeting with his North Korean counterpart Kim Kye Gwan during a recent stop in China but that the North rejected it. South Korean officials said they couldn't confirm the report.
North Korea has demanded direct talks with the U.S., but Washington had refused, saying it would only speak to the North in meetings with the other countries involved in the talks.
Washington eased a half-century of economic sanctions against North Korea in 1999 in exchange for Pyongyang establishing a moratorium on long-range missile tests, one year after the North stunned the world by launching a long-range missile over Japan.
The North broke that self-imposed moratorium with its July launches, which included a long-range missile believed potentially capable of reaching parts of the U.S. The missile exploded shortly after takeoff.
The launches drew international objections and prompted the U.N. Security Council to unanimously adopt a resolution condemning its actions.
The resolution bans all member states from transactions with North Korea involving material or technology for missiles or weapons of mass destruction. It also says that countries should take steps to prevent the transfer of any financial resources that could be connected to the North's weapons programs.
The South Korean official's statements Tuesday were the first indication that the U.S. was moving toward sanctions.
"The United States will slap sanctions on the North," the official said. He added that Washington has already advised countries including South Korea that they should implement the U.N. resolution by enacting sanctions.
South Korea, which had voiced its opposition to sanctioning its northern neighbor and largely maintained a policy of engagement, will "do what it needs to do," the official said, indicating Seoul will join the sanctions.
On Monday in Seoul at the end of his Asian tour, Hill proposed a meeting of North Korea's neighbors and other regional powers on the sidelines of the upcoming meeting of the U.N. General Assembly, South Korean officials said.
The U.S. has moved to sever North Korea's connections to international banks, alleging any transactions by the Pyongyang regime are suspect and could be connected to illegal activity — including counterfeiting of U.S. dollars and money laundering related to the sales of weapons of mass destruction.
Those restrictions prompted the North to boycott the six-nation nuclear talks since last year and demand the U.S. retract the measures. Washington has refused and said the issue is unrelated to the nuclear standoff.
South Korea's main spy agency has said the communist regime could test a nuclear device at any time. South Korean Defense Minister Yoon Kwang-ung told a parliamentary meeting Monday that a nuclear test by the North remains a possibility. He didn't elaborate.
Many experts believe the North has enough radioactive material to build at least a half-dozen nuclear weapons.