WASHINGTON (AP) — North Korea is upgrading one of its two major missile launch sites, apparently to handle much bigger rockets, and some design features suggest it is getting help from Iran, a U.S. research institute said Thursday.

A successful satellite launch in December, and a nuclear test on Tuesday, both in defiance of U.N. Security Council resolutions, have intensified concern that North Korea is moving toward its goal of building a bomb small enough to be fitted on an intercontinental missile.