By Andrea Shalal-Esa

WASHINGTON (Reuters) - Lockheed Martin Corp's (LMT.N: Quote, Profile, Research) presidential helicopter program is significantly over budget and will breach a cost threshold set by Congress, although the extent of the cost overruns are not yet clear, the Pentagon's chief arms buyer John Young said on Tuesday.

Costs in the first part of the VH-71 helicopter program rose $1.4 billion to $3.7 billion, Young told reporters. Costs in the second phase now were on course to be $3 billion higher than expected, and had reached $7.5 billion, he said.

That would bring the overall cost of the program to $11.2 billion, up from an initial estimate of $6.1 billion.

Young said he was working with the White House, the Navy -- - which oversees the Marine Corps program -- and Lockheed to find a way to rein in costs.

Young said problems with the program, which started when he was running Navy acquisition programs, were not due to extra White House requirements, as some sources have said.

Instead, he said that neither the government nor Lockheed had understood the full scope of the requirements and their implications for the helicopter or costs.

In January, top Pentagon, Navy and Lockheed officials reviewed options for restructuring the program, including cutting the program significantly, or replacing the Lockheed helicopter with a modified H-3 helicopter built by Sikorsky Aircraft, a unit of United Technologies Corp (UTX.N: Quote, Profile, Research).

Officials agreed to proceed with the Lockheed helicopter, but they are still working out details of how much it will really cost to meet the program's requirements.