It's pretty unusual for all 100 members of the Senate to meet for an event like this at the White House. But Majority Leader Mitch McConnell convened the meeting; the White House is just hosting it.
Earlier in the day, President Trump discussed the situation in a phone conference with China's President Xi Jinping and Japanese Prime Minister Shinzo Abe.
Xi told Trump his country was opposed to North Korea's nuclear program and urged all parties to keep cool heads.
Reuters reports that House members are seeking a similar briefing.