Garold Larson, deputy U.S. ambassador to the Conference on Disarmament, also urged the NPT's near-global membership to deter violators from withdrawing from the treaty in the future.
North Korea left the NPT in 2003 and exploded a nuclear test device in 2006. It is now dismantling its nuclear arms program under a six-power pact. Iran is under U.N. sanctions over suspicions that it is secretly preparing to build atomic bombs.
Speaking to a U.N. seminar in Geneva celebrating the 40th anniversary of the NPT, which aims to prevent the spread of nuclear weapons, Larson said the past two decades had witnessed the proliferation and misuse of nuclear technology.
"The United States remains very concerned that parties like Iran have violated their commitments and thereby undermined the treaty," he said.
"Parties should take concrete action to confront and deter noncompliance and encourage violators to come back into compliance."
A offer by major powers to Iran of incentives and a halt to sanctions if it shelves its nuclear activities has not won over Tehran. It says it is enriching uranium only for civilian atomic power. The West believes Iran's underlying goal is nuclear arms.
Larson said NPT states should also identify steps to deter future withdrawals from the treaty by violators and ensure that their "misuse of the benefits of the NPT do not endanger the security" of others.
The pact has been ratified by 189 states, including North Korea.
Larson did not refer to U.S. intelligence findings that Syria covertly tried to build an atomic reactor with North Korean help at a site bombed by Israel last year. Syria denies the accusations.
Investigators from the U.N.'s International Atomic Energy Agency said after a four-day visit to Syria last week that they had examined the site but that more checks were needed.
Unlike Syria, Israel has not signed the NPT. It is widely believed to have the only nuclear arsenal in the Middle East. India and Pakistan also have nuclear arsenals and have stayed outside the NPT.
Peacemaking in the Middle East, South Asia and Northeast Asia was the "surest pathway to reach NPT universality", Larson added.
(Reporting by Stephanie Nebehay; editing by Mark Heinrich and Andrew Roche)
