The Reagan administration, seeking to block the threat of Soviet expansion in regions it considers vital to U.S. interests, plans to announce today a military assistance program that will include big increases in security aid for El Salvador and several countries in the Middle East.

Despite President Reagan's determination to make major budget cuts in domestic programs and nonmilitary foreign aid, informed sources said yesterday that the security assistance program to be presented to Congress today will call for outlays greater than those in former president Carter's proposed budget for fiscal year 1982.

It would increase about $1 billion above current levels in foreign military sales credits, much of this more highly subsidized than in the past; about $400 million in security-related economic support funds that are cash payments to help governments balance their budgets, and $15 million more in training for foreign armed forces.

For El Salvador, which Reagan and Secretary of State Alexander M. Haig Jr. have made the first test of their campaign to halt communist expansion in the Third World, the administration will ask Congress to appropriate $25 million in credits for purchase of weapons and military equipment, $1 million for military training and $40 million in economic support funds.

Those amounts will be in addition to the $25 million in emergency military aid granted to El Salvador last week. Only a year ago, when the fiscal 1981 budget was passed, that Massachusetts-sized Central American country was receiving only $5.4 million in security assistance.

While the figures for El Salvador represent on their face the most dramatic jump, the sources said the biggest increases are directed at the Middle East and Persian Gulf region, where Haig has declared that the United States faces "a deteriorating security situation" and a "growing threat" from the Soviet Union.

According to the sources, the administration's fiscal 1982 proposal will call for total foreign military sales credits of about $4 billion and $2.6 billion in economic support funds. Of that overall amount, the sources added, $2.5 billion of the sales credits and $1.6 billion of the support funds are earmarked for the Middle East.

For the current fiscal year, Congress had appropriated $3 billion in credits and $2.2 billion in support funds, with the Mideast subtotals standing at $2.3 billion and $1.9 billion, respectively. The Carter proposal for fiscal 1982 had called for a total of $3.8 billion in credits and $2.4 billion in economic support funds.

In addition, the sources said, the Reagan proposals contain some ideas that are likely to prove very controversial in Congress.

One is a request for a general economic support fund of almost $500 million to be used in unforeseen emergencies. Some sources described it as "tantamount to a free-floating slush fund that Haig could use to pump money into El Salvador or any other country without Congress having any say over its use."

Another proposed feature would give friendly countries the opportunity to buy weapons and equipment on especially favorable concessional terms or by forgiving repayment of a large part of the credit loans.

In addition to Israel and Egypt, which have had such favored treatment, the sources said the administration also wants to apply it to the Sudan, which it regards as threatened by Libya, and to Kenya and Somalia, two African countries on the Indian Ocean where the United States has obtained base-use rights for maintaining a greater military presence in the area.

The major recipient will be Israel, for whom the administration is asking $1.4 billion in credits, with $500 million to be forgiven, and $785 million in support funds, of which two-thirds will be a grant. The request for Egypt involves $900 million in credits, with $400 million of that in direct credits that amount to a grant and $750 million in support funds.

Other proposals involving the Mideast and Indian Ocean regions include Jordan ($50 million in credits and $20 million in support funds), the Sudan ($100 million in credits and $50 million in support funds), Kenya ($51 million in credits) and Morocco ($30 million in credits).