The Carter administration yesterday released a letter to Israel suggesting that the United States will go far beyond just providing money to help Israel build two air bases in the Negev Desert to replace those to be lost in the Sinai under the Camp David accords.

Administration sources said Israeli officials have been expressing doubts about finishing the replacement bases within the three years provided by the accords unless the United States pitches in with more than money.

The Sept. 28 letter, released yesterday, from Defense Secretary Harold Brown to Israeli Defense Minister Ezer Weizman states that the United States understands "the special urgency and priority which Israel attaches to preparing the new bases in light of its conviction that it cannot safely leave the Sinai air bases until the new ones are operational."

Given President Carter's determination to see the accords implemented on schedule and Israel's newly expressed doubts about meeting the deadline for vacating the Sinai air bases, the U.S. role in the Negev is likely to be larger than the banker-consultant one originally portrayed by administration spokesmen.

In contrast to earlier statements that the actual construction would almost certainly be done by private contractors working for the Israeli government, one administration source said yesterday that, given the time urgency, "It's not inconceivable" some specialists from the U.S. Army Corps of Engineers would end up on the Negev to speed construction of the air bases.

The Pentagon's direct involvement will grow next month when a team of its experts will go out onto the Negev to study the ground and also collect data about Israel's Sinai bases at Eitan and Etrion.

"The terrain of the Negev is very difficult," Harold H. Saunders, State Department Mideast executive, told a House International Relations subcommittee Thursday.

Israeli officials have been stressing lately that their facilities at Etzion go beyond just landing and maintaining warplanes, and include a vast intelligence complex that will be difficult and costly to move.

Negev will cost is still guesswork,

How much the new air bases in the with Pentagon officials sticking to their original wide-ranged estimate of from between $150 million to $500 million for each base.

The Brown letter to Weizman said that Carter "is prepared" to submit a special request to Congress for air base money, but does not state how much is likely to required.

Information sheets circulating within the government note that how fast that money will be needed depends heavily on whether both airfields are built at once on the Negev or one after the other.

Brown's letter was released as U.S. envoy Alfred Atherton met Prime Minister Menachem Begin in Jerusalem about the timetable for negotiations.

Under the Camp David framework, Egypt will use the Sinai air bases only for commercial aviation, not military.