United Continental resumed service Wednesday after grounding its flights worldwide for two hours because of a computer system problem.
The worldwide groundstop had impacted about 3,500 flights, the airline said, apologizing to customers "for any inconvenience."
"An issue with a router degraded network connectivity for various applications, causing this morning's operational disruption," United said. "We fixed the router issue, which is enabling us to restore normal functions."
Earlier, it directed customers to united.com to change their flight plans.
The Port Authority of New York and New Jersey said it was working to assist passengers whose flights were disrupted at New York City's three major airports.
"Our airports are assessing/monitoring the impacts of this problem and are working with United to accommodate flights in other terminals and assist passengers to the extent possible," it said.
On June 2, the airline cited "automation issues" for the grounding of some of its flights.