Washington (CNN) Adm. Harry Harris, who oversees US military operations in the Indo-Pacific, said Thursday that "we can't be overly optimistic on outcomes" when it comes to the recently announced summit between US President Donald Trump and North Korean leader Kim Jong Un.

"As we go into this, I think we can't be overly optimistic on outcomes. We'll just have to see where it goes if and when we have the summit," Harris told a hearing of the Senate Armed Services Committee, adding that he believed the complete and verifiable denuclearization of the peninsula would remain the goal of the talks.

Speaking from the White House earlier this month, South Korea's national security adviser made the surprise announcement that Trump had accepted an invitation from Kim to meet, saying that the summit would take place by May. The White House later confirmed Trump's acceptance of the invitation.

Harris said he was "encouraged by recent developments on the Korean Peninsula and the possibility of a summit between President Trump and Kim Jong Un" but added that "North Korea remains our most urgent security threat in the region."

"This past year has seen rapid and comprehensive improvement in North Korea's ballistic missile and nuclear capabilities despite broad international condemnation and the imposition of additional United Nations security resolutions," Harris said.

Read More