Defense Secretary Harold Brown told Saudi Arabia today that the United States is willing to sell jet warplanes to North Yemen and Sudan provided the Saudi government pays the bill.

The agreement in principle to supply advanced weapons to the two Arab nations was the first tangible result of a new U.S. pledge to consult more closely with Saudi Arabia on defense matters.

U.S. officials said Brown delivered to Saudi Crown Prince Fahd a personal letter from President Carter to King Khalid offering periodic consultation between Washington and Riyadh on matters affecting the security of the Persian Gulf region.

One American official said Brown was prepared to discuss specific measures the United States would be willing to take as part of the general commitment. But he said the Saudis did not ask for details so Brown did not supply them. The official did not reveal the details Brown was prepared to discuss to reporters.

"My sense is they [the Saudis] are utterly delighted but they have not quite come to grips with it," said the official, who declined to be quoted by name.

The official said Saudi Arabia has been urging the United States for months to take a firmer stand in the region to counterbalance Soviet influence in Afghanistan, South Yemen and Ethiopia.

Saudi Commerce Minister Suleiman Solaim said in an interview that the Riyadh government considered it very significant that Brown made public the U.S. pledge to help Saudi Arabia resist outside aggression.

Solaim said that U.S. presidents since Harry S. Truman have made such promises. But the pledges were always kept secret, apparently to avoid angering Israel's supporters in the United States.

"What used to be secret messages from your president and your secretary of state, we are now hearing from the mouth of your secretary of defense. What is new is that it is being said in public," Solaim said.

In return, Solaim said Saudi Arabia is prepared to play a moderating role in the Arab world and in the Organization of Petroleum Exporting Countries. But he cautioned against the "unrealistic expectation that his country would force the Camp David peace formula down the throats of other Arabs or achieve a rollback in oil prices.

U.S. officials revealed the discussions concerning weapons sales for North Yemen and Sudan aboard Brown's plane on the hour-long flight from the Saudi capital of Riyadh to the Gulf coast city of Dhahran. Brown is also scheduled to visit Jordan, Israel and Egypt.

The officials said the United States is prepared to sell 12 F5 fighter-bombers to North Yemen and is willing to consider selling additional F5s, tanks and armored personnel carriers. The official said the package would total about $300 million, with the Saudis providing the money.

The deal would be subject to congressional approval and the official cautioned that no final decisions have been made.

Saudi officials said they generally were very pleased with Brown's public expression of support, although it contained no specific measures. But these same officials said they fear that pressure from pro-Israeli groups might yet upset the relationship.

Solaim was especially critical of Senate Foreign Relations Committee Chairman Frank Church (D-Idaho), who recently attacked Saudi Arabia for failing to urge Arab acceptance of the Camp Cavid peace plan. Solaim said Saudi Arabia is urging Morrison-Knudsen, a giant construction firm based in Boise, Idaho, which does millions of dollars worth of business in Saudi Arabia, to "tell him to watch out."