Proliferation experts widely agree that Israel is the only country in the Middle East with atomic weapons, but it refuses to confirm or deny if it has a nuclear arsenal.
Rose Gottemoeller, the US assitant secretary of state for verification and compliance, said at the United Nations on Tuesday that encouraging all countries to sign up to the agreement was a major goal for Washington.
"Universal adherence to the NPT itself, including by India, Israel, Pakistan and North Korea ... remains a fundamental objective of the United States," she said.
Gottemoeller did not say whether Washington planned new steps to press Israel on the issue.
The pact has been signed by 189 nations, five of which have declared nuclear weapons - the US, Britain, France, Russia and China.
Of the non-signatories, India, Pakistan and North Korea have all openly tested nuclear weapons.
Washington recently signed a civilian nuclear co-operation deal with New Delhi despite the fact that it has not agreed to the pact.
Parties to the NPT have a recognised right of access to peaceful uses of nuclear energy and an obligation to co-operate on civilian nuclear technology.