WASHINGTON - The US Marine Corps is buying 80 F-35C joint strike fighter planes, the Pentagon said, casting doubt on plans to buy a vertical-landing version still under development.

A short take-off, vertical landing model the F-35B destined for the Navy has run into "considerable" development difficulties and has been given two years by Defense Secretary Robert Gates to resolve the issues or risk the program being scrapped.

On Monday Chief of Naval Operations Gary Roughead and Commander of the Marine Corps General James Amos signed a protocol to buy 80 F-35C aircraft destined to equip aircraft carriers, the Marine Corps said in a statement.

The accord "incorporates a plan to provide five US Marince Corps squadrons of F-35C Joint Strike Fighter (JSF) aircraft to the Navy's carrier air wing," the statement said.

But it stressed "it is important to note that the continued development of F-35B remains the centerpiece" of the Corps's tactical fixed wing modernization program.

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"The Marine Corps remains unequivocally committed to the success of the F-35B program," said spokesman Major Joseph Plenzler in the statement.

"Our hope was that we would be able to someday fly those versions off of naval aircraft carriers. In the meantime, it would seem prudent that we would buy some number of C variants (even early on) so that we can begin to transition our force," he said.

Plenzler said the US currently has 11 carriers and 11 large-deck amphibious ships, flying tactical fixed wing aircraft.

"A couple years from now, with F-35Cs on board 11 aircraft carriers and F-35Bs flying off of 11 large-deck amphibious ships, our nation would have, for the very first time, 22 capital ships with fifth-generation aircraft flying off of them," he added.