The Centers for Disease Control and Prevention (CDC) has confirmed a third measles-related death in the United States, as the country faces its largest measles outbreak in decades.

The CDC announced the death on Tuesday but did not specify in which state it occurred. The agency confirmed the fatality after the National Center for Health Statistics (NCHS) verified that measles was listed as the underlying cause of death on a death certificate.

The CDC reported 4,410 measles cases nationwide as of October 15. The agency has cautioned that its death count could change as further reviews are completed.

The federal tally remains lower than figures reported by state officials. Pennsylvania, the state hardest hit by the outbreak, had reported 1,286 cases and five deaths as of Monday.

The outbreak in Pennsylvania has spread well beyond its initial epicentre in Lancaster County, with 42 counties now reporting cases.

The federal government's approach to counting measles deaths has drawn scrutiny. The CDC shifted to relying on mortality data from the NCHS rather than on reports from state health departments, a change that has raised questions about the accuracy and consistency of the national count.

The CDC has said it is working with the Council of State and Territorial Epidemiologists to establish a standardised definition for measles deaths. In the meantime, the agency is using NCHS mortality data as its measure.