A former senior U.S. official on Monday said North Korea's recent threats against South Korea appear to be a test of the Seoul-Washington alliance and an attempt to open up more gaps between the allies.

Christopher Hill, former Assistant Secretary of State for East Asian and Pacific affairs, made the assessment at a virtual seminar hosted by the International Crisis Group.

Hill, who also served as the chief U.S. envoy to the six-party nuclear talks, projected Pyongyang wouldn't proceed with military retaliation against Seoul, saying what it is doing is more political in trying to humiliate the South.

He then stressed the need to further solidify the alliance, saying Trump's obsession with increasing Seoul's share of defense costs is hampering a coordinated response to North Korean threats.

Robert Gallucci, a former chief U.S. negotiator, who produced the Agreed Framework with North Korea in 1994, supported a step-by-step and action-for-action approach in negotiating denuclearization with the North.

While not excluding summit-level talks, Gallucci said progress should first be made through working-level negotiations.