WASHINGTON (AP) _ The Gramm-Rudman deficit-reduction law will cut in half the military aid available for key countries such as the Philippines, the chairman of the House foreign operations subcommittee says.

Only four countries - Israel, Egypt, Pakistan and Northern Ireland - would be protected from cuts and receive the full amounts requested by President Reagan, Rep. David Obey, D-Wis., said Wednesday.

Obey was briefing reporters on the eve of a closed-door session of his appropriation subcommittee that will draw up its version of the 1987 foreign aid bill.

He said that under the Gramm-Rudman law, the budget resolution passed last month means a cut of nearly $3 billion from the administration's proposed package totaling $15.5 billion.

The administration has proposed $3 billion for Israel, $2.1 billion for Egypt, $669 million for Pakistan and $50 million for Northern Ireland.

Obey said Israel and Egypt, which receive the largest amounts of U.S. aid by far, would be protected because of widespread support for the Camp David peace process as well as the desire to support Israeli Prime Minister Shimon Peres in the new Middle East initiative with Moroccan King Hassan II.

Aid for Pakistan has strong support because of the Soviet occupation of neighboring Afghanistan, and Irish aid proved popular when it was proposed by the administration earlier this year to support the new Anglo-Irish accord to bring peace to the troubled British province.

Obey said most of the cuts he proposes are for general funds shared by a number of countries. Since the four countries are protected, the cuts borne by other countries sharing their funds will be even more drastic than if the reductions had been across the board.

In two key general funds, his version would mean 51 percent less in loans for arms sales and 32 percent less for economic aid, he said.

The administration originally proposed $95 million in economic aid and $50 million in arms loans for the Philippines and has already sought more for this year to support the new government of President Corazon Aquino.

If the administration wants to provide the full amount it requested for the Philippines, it will mean even less for the remaining countries, Obey explained.

Some 120 nations receive aid from the United States.

''I have been absolutely even-handed,'' said Obey, adding that he was recommending military loans 16 percent lower than when he took over the subcommittee two years ago, economic aid 16 percent lower and development assistance 15 percent lower.

''My main job as chairman is an educative job,'' trying to explain how the Gramm-Rudman law passed last December forces the administration to make cuts in areas that it wants to increase, he said. The law is designed to reduce the federal budget deficit to zero by 1991 by imposing across-the-board spending cuts if Congress and the president can't hold the line voluntarily.

Obey said he opposed the legislation at the time, but that he was going to adhere to its provisions in bringing the foreign-aid bill to the floor of the House.

''This is not an Obey plot,'' he said. ''This is what Gramm-Rudman requires.''

He said that although he proposed giving the administration flexibility for most cuts, he was moving to keep officials from using Gramm-Rudman to eliminate items such as the Peace Corps and support for UNICEF that are popular with Congress but not with the administration.

In those cases, he said, his proposal requires a certain expenditure for the controversial items.