The Carter administration yesterday said it would resist congressional pressure to go beyond an expansion to $5.8 billion in the Export-Import Bank's financing resources for the current, 1980 fiscal year, and flatly denied the charge that the bank "has been less competitive" than its foreign counterparts in supporting American exporters.

C. Fred Bergsten, Treasury assistant secretary for international affairs, said that the administration is deeply committed to helping exporters, but is constrained by the need to limit the federal budget deficit.

In the course of detailed hearings on the Ex-Im Bank's programs and budgets, John L. Moore Jr., president of the bank, also revealed that it is "unlikely" that any Ex-Im credits will be authorized to China in the balance of this fiscal year. Estimates of such loans in fiscal 1981, starting Oct. 1, "would be highly speculative," he added.

Vice President Mondale, when in China last August, said that the Ex-Im Bank was ready to establish credit arrangements for China, or a case by case basis up to a total of $2 billion over a five-year period.

Moore said yesterday the U.S. is waiting for China to make decisions on its borrowing program, and that the delay will not put U.S. exporters at a competitive disadvantage with those in other countries that already have extended a total of $16.3 billion in export credits.

Separately a U.S. Chamber of Commerce team that recently returned from a two-week visit with officials in China, said that the Export-Import Bank issue -- now that the symbolic commitment had been made -- is not a problem. "The Chinese have more credits than they know what to do with," said L. Oakley Johnson of the Chamber's international division, "and the U.S. is close to or on top of the list of China's best customers." "

At the hearing, Christopher H. Phillips, president of the National Council for U.S.-China Trade, argued that an early understanding on the credits would put U.S. companies "on an equal footing" with their major trading competitors for the first time.

On the overall question, Bergsten conceded that foreign competition had been increasing in the form of "mixed credits" -- export aid and mixed with aid credits that reduce the overall cost to the borrower.

But he claimed that the U.S. has been able to meet that "noxious practice" on a selective basis, citing a "major scale" that the U.S. had taken away from Britain in Cyprus.

Nonetheless, Bergsten's presentation was greeted with skepticism by members of the Senate Banking subcommittee on international finance, notably Adlai Stevenson (D-Ill), John Heinze III (R-Pa.) and Jake Garn (R-Utah).

In general, the senators argued that U.S. industry confronts an unfair trade advantage because other governments more generously support their export industries.

As compared with the $58 billion planned for Ex-Im this year by the administration -- which includes $4.1 billion in direct loan authority and additional "off-budget" monies as standby or guaranty resources -- Heinz called for a boost to $9 billion.

The complaining senators were backed by a General Accounting Office report which said the Export-Import Bank had only a limited capability to match some of the assistance programs of foreign governments. Frank C. Conahan, senior associate director of GAO's international division, testified that a "reexamination of the framework and financial constraints within which Ex-Im Bank now operates seems indicated."

Bergsten said that increasing pressures on other major countries to limit their subsidies are bearing fruit. Moreover, he said, the expansion in Ex-Im funds, from only $700 million in fiscal 1977 to $5.8 billion in fiscal 1980, has allowed U.S. exporters "to compete fully with foreign exporters."

He cited Ex-Im data showing that of 143 credit transactions supported in 1979, only one was lost because the bank offered less favorable terms than some other officials agency.