WASHINGTON, June 5— Two Saudi Arabian fighter pilots shot down two American-built Iranian F-4 fighters today, Saudi and United States officials said.

The Saudi pilots were flying American-built F-15's and had help from United States aerial tanker planes and surveillance planes, they said.

In London, President Reagan was said to be concerned that the Saudi air action could intensify the fighting.

In Kuwait, Western and Arab officials described the aerial fight as a widening of the conflict. Iran said 600 people were killed in an Iraqi air raid on the town of Baneh, but did not make any immediate comment on the aerial clash with the Saudis. (Page A3.)

Three More Radar Planes on Way

At the same time, the Defense Department announced that three additional surveillance planes were being sent to Saudi Arabia. One such plane was sent out last weekend.

The Saudi Ambassador, Prince Bandar bin Sultan, who is a fighter pilot himself, said at his embassy here: ''Our sovereignty was violated and we reacted, as we said we would all along, in a defensive manner.''

In the aerial engagement today, F-15's, the United States Air Force's most powerful fighters, were used by the Saudis against Iran's F-4's, fighter- bombers of Vietnam War vintage, and fired American-made air-to-air missiles to bring down the Iranian planes. Saudi Position Is Stated

Prince Bandar, who is a son of the Saudi Defense Minister, said:

''We think it is a pity we had to be dragged into this conflict. We are determined to defend our country. People should not mix up moderation with consent. We do not consent, nor do we find it amusing to be attacked or for our interests to be attacked.''

He was apparently alluding to Saudi diplomatic efforts to persuade Iran to cease its attacks on shipping in the Persian Gulf.

A Middle Eastern diplomat said the aerial fight occurred in Saudi airspace near an islet named Al Arabiyah, about 60 miles northeast of Jubail. The Saudi planes jumped the Iranian planes as they prepared to attack two ships and shot them down with missiles, he said.

Shortly after, Iran sent up 11 more F-4's that were picked up on radar, he said. In response, Saudi Arabia put 11 more F-15's into the air. After a brief standoff, the Iranian fighters broke off, and the Saudis returned, the diplomat said. American officials and Middle Eastern diplomats said a United States flying tanker had refueled the two Saudi planes while they were on patrol before the aerial fight while an American radar plane provided the information that enabled the Saudi fighters to find the Iranian planes.

In a related development, the Defense Department's chief spokesman, Michael I. Burch, said at a briefing that United States warships would fire at planes or ships that threatened American oilers carrying fuel from the gulf to Navy ships in the Arabian Sea.

''We will do everything possible to protect U.S. vessels,'' he said.

Mr. Burch, in response to questions, denied that American warships were escorting Kuwaiti or American merchant ships in the Persian Gulf. But he said the four warships in the gulf were keeping the Navy's oilers under surveillance.

The Middle Eastern diplomat said the Saudi Air Force, which has 61 F-15's, began flying continuous daylight air patrols after a United States Air Force KC-10 aerial tanker arrived early last week. Intermittent Night Patrols

He said the Saudis were also flying intermittent patrols at night. The KC- 10 tanker, along with three KC-135 tankers already in Saudi Arabia, make possible longer patrols by refueling planes in flight. Saudi Arabia claims airspace extending 12 miles from its shores. Since the island of Al Arabiyah is considered Saudi territory, the country also claims the airspace in the 12 miles surrounding the island.

As recounted by the diplomat, the two Saudi planes were on patrol when they were alerted that two Iranian F-4's, flying from a base at Bushire, were streaking toward two ships.

The Saudi fighters headed for the Iranian jets, which turned away and flew just above the water. The Saudis pursued and, with one missile each, shot down the Iranian planes. Initial radar reports showed only one Iranian plane destroyed, but videotapes from the Saudi planes later showed two.

About 30 minutes later, the Saudi air defense system, into which information from the American radar planes is fed, reported that 11 more jets had taken off from Bushire and were flying across the gulf. The Saudis responded by putting up their 11 jets, but no fight ensued.

The Pentagon spokesman, Mr. Burch, indicated that American radar planes were not in danger of being attacked since they remain over Saudi Arabia and out of Iranian fighter range. Moreover, the radar planes can spot potential attackers 200 miles away and fly out of their range.

The dispatch of four United States radar planes equipped with maritime detection devices began last weekend when the first was sent to Saudi Arabia to replace one of the four older models on duty there. Newer Models More Advanced

The newer radar planes, of which the United States has 10 in a fleet of 34, can monitor surface ships, both moving and still, and distinguish between a large tanker and a small warship.

The older versions are unable to spot ships. Neither version can spot vehicles on land unless they are moving more than 80 miles an hour.

The radar information is fed into computers for collation and transmitted to a ground station. In Saudi Arabia, the radar planes send their data to an American ground station, which relay it to a Saudi operations center. So far, officials said, the radar planes have not communicated directly with Saudi fighters.

Mr. Burch declined to discuss the effect of the Saudi deployment on United States military operations elsewhere. One or two of the maritime versions are used in training, according to Pentagon officials, while others are on duty in Iceland to watch the North Atlantic and still others in Okinawa to watch the Western Pacific.

Reagan Reported Concerned

LONDON, June 5 - President Reagan was described today as concerned that the downing of Iranian jets by Saudi fighters, with the assistance of a United States radar plane, could widen the war in the Persian Gulf.

A senior Administration official said at a briefing that the Saudi action had been in legitimate self-defense.

''The President was concerned to the extent that it represented an escalation in tension and in violence,'' the official said. He added that Mr. Reagan was ''conscious that there was no apparent requirement for U.S. assistance of any kind, or involvement, other than to caution restraint on both sides.''

map of Iran (page A4)