The chief of the U.S. Urban Mass Transportation Administration declined yesterday to commit the Carter administration to support the full completion of Metro's planned 100-mile subway system.

Richard S. Page, UMTA administrator and chairman of an administration task force that is studying Metro's financial problems, said that "we would review" any regionwide proposal for completing all of the system "and then make a determination of what we would recommend to the Congress."

Page also told the House District of Columbia Subcommittee on Economic Development and Regional Affairs that UMTA can identify about $800 million that could be used for Metro construction. That would leave Metro about $1 billion short of the presented estimated cost for completing the system. Total cost is now estimated at about $6 billion.

Page was testifying along with Metro board chairman Joseph S. Wholey and Metro general manager Theodore C. Lutz at the first session of the subcommittee's oversight hearings on Metro and its finances.

Rep. Herbert E. Harris II (D-Va.), chairman of the subcommittee, repeatedly tried to pin the witnesses down on precisely when they will complete work on the plans for finding money for the rest of Metro.

Wholey, who heads the committee that is drafting the plan, has said that he will submit it to UMTA by Aug. 31. Page said yesterday UMTA will complete its review "within 30 to 45 days" after that.

After some wheedling back and forth, Page said he would try to get it done within two weeks. Federal officials have been attending almost every committee meeting, following the development of that plan, Wholey pointed out.

The plan, under federal directive, must outline where the money will be found that will make it possible to build beyond the 64 miles of Metro that presently are funded or agreed to, as well as the source of money that will pay the interest on and retire $1 billion in revenue bonds sold to build Metro and to pay for the operating costs of Metro not met by fares.

Metro presently collects about half of its operating costs from the fares. Most of the rest is made up from local property tax revenue.

Wholey was asked whether the metropolitan area can afford to build 100 miles of Metro and can further afford to operate that much of it. He said yes to both questions. 'I'm confident the local governments, if allowed to stick together and not be driven apart by the federal government, will come up with our share" of the construction costs, Wholey said.

Wholey told the subcommittee that, if UMTA requires Metro to indicate acceptance of a shorter, interim subway system, that process will be "politically divisive. To suggest to Fairfax County or Montgomery County that they should keep paying in but won't get their line . . . is very difficult," he said.