CAPE CANAVERAL, Fla. (Reuters) - The Russian-made rocket motor that catapulted a United Launch Alliance booster toward orbit last week shut down six seconds early apparently because of a fuel system problem, the company said on Thursday, in its first explanation of the issue.

The ULA Atlas 5 rocket blasted off from Cape Canaveral Air Force Station in Florida on March 22 carrying an Orbital ATK cargo ship bound for the International Space Station.

The rocket’s Russian-made RD-180 engine shut down about six seconds early, but the booster’s second-stage motor compensated for the shortfall by firing longer, ULA said in a statement.

ULA is a joint venture of Lockheed Martin Corp and Boeing Co.

Orbital’s Cygnus cargo ship arrived at the space station on Saturday, as scheduled, despite the problem.

ULA said preliminary indications pointed to a problem with the rocket’s first-stage fuel system and related components, but the investigation was continuing to find the root cause and identify appropriate solutions before any further flights.

The U.S. Air Force, which is ULA’s primary customer, is participating in the review, ULA said.

The company last week delayed its next Atlas 5 launch by at least a week while analysis is underway.