He said the president and Vice President were at the briefing, and the president gave an introduction.
Sen. Marco Rubio (R-FL) called it an "important" meeting.
North Korea's UN Mission has said the government will react to "a total war" with the United States with a nuclear strike. Harris will testify before a Senate committee on Thursday, while Secretary of State Rex Tillerson will chair a UN Security Council meeting on North Korea in NY on April 28. Secretary of State Rex Tillerson, Secretary of Defense James Mattis, and Chairman of the Joint Chiefs John Dunford delivered what's been described as a "sober" briefing that "lacked straight answers".
It added: "We are engaging responsible members of the global community to increase pressure on the DRPK in order to convince the regime to de-escalate and return to the path of dialogue".More news: N'Golo Kante: Why Chelsea midfielder won PFA Player of the Year
The United States is "vigilant" about the possibility of another North Korean nuclear test, or ballistic missile test, or intercontinental ballistic missile test, and is "prepared to respond to any of those provocations", the official said. "We remain open to negotiations towards that goal".
It is created to intercept and destroy short and medium-range ballistic missiles. The regime then launched four ballistic missiles on March 6. It also launched a long-range rocket previous year that delivered a satellite into orbit, which Washington, Seoul and others saw as a banned test of missile technology.
South Korea's navy has said it will hold drills with the USA strike group.
The United States and North Korea have stepped up warnings to each other in recent weeks over North Korea's development of nuclear weapons and missiles in defiance of U.N. resolutions.More news: Canada introduces legislation to legalise marijuana
South Korea's trumpeting of progress in setting up the Terminal High-Altitude Area Defense system, or THAAD, comes as high-powered USA military vessels converge on the Korean Peninsula and as a combative North Korea signals possible nuclear and missile testing. They were ferried from the Capitol by bus. Tillerson will chair a ministerial meeting of the U.N. Security Council on Friday that is expected to discuss tougher steps, which US officials say could include an oil embargo, banning North Korea's airline, intercepting cargo ships and punishing Chinese and other foreign banks doing business with Pyongyang.
"What the president has done is he's made a decision for us to pursue a certain course", the official told reporters in the White House briefing room, "and that course obviously has a number of options associated with it depending on how the situation develops in the future".
"I really don't understand what is the president's game plan to get North Korea to change its calculation", Senator Ben Cardin, the ranking Democrat on the Senate Foreign Relations Committee, told reporters. Ed Markey (D-MA) said on NBC News it was "heartening" that there was more talk about diplomacy, but said it was in the context of more US military exercises in the region and expressed concern about an "accidental war".
Kim Jong Un's regime is testing ballistic missiles and nuclear devices with growing frequency and aggressiveness, Admiral Harry Harris said in testimony Wednesday before the House Armed Services Committee.More news: Industry Cheers Perdue's Confirmation as USDA Secretary
Sen. Ted Cruz (R-TX) after the meeting tweeted that it was "time to re-list" North Korea as a state sponsor of terrorism, following a Reuters report saying that the administration was considering doing so.