President Reagan said yesterday the United States would "go to the source" if foreign governments are found to be sponsors of terrorist acts. But he declined to comment on a weekend press report that the administration privately threatened Iran with reprisals last month if any of the Americans kidnaped by pro-Iranian groups in Lebanon are put on trial or executed.

"There is increasing evidence that some terrorists in the world are actually emissaries of sovereign governments," Reagan said in an interview with The Washington Post. "And if that's the case and it can be established, then that business of trying to find and track down in all the world a few terrorist individuals for some crime -- no, go to the source, the government that supports them."

Reagan, however, pointedly refused to confirm or deny a Sunday Times of London report that the White House threatened military retaliation against Iran in a message conveyed by the Swiss diplomatic mission in Tehran.

The Sunday Times article, quoting unnamed western intelligence sources, said that the message did not specify what action might be taken but that American intelligence officials had been asked to recommend suitable Iranian targets.

Among the possible targets were Iran's main oil export terminal at Kharg Island in the Persian Gulf, its emergency oil export terminal at the Sirri Islands closer to the mouth of the gulf and its main commercial ports, Bushehr and Bandar Khomeini, according to the Sunday Times.

"I don't think I should discuss anything of this kind," Reagan said when asked about the dispatch.

While Reagan was elliptical in his comments, national security adviser Robert C. MacFarlane last week had specifically tied terrorists "responsive to Iranian guidance" to attacks on U.S. citizens, property and interests, and had advocated then a proportional military response against military targets in states that direct terrorist actions against the United States.

An organization calling itself Islamic Jihad and believed to be a group of Shiite Moslem extremists with strong Iranian connections has claimed responsibility for the abduction of five Americans who are missing after being kidnaped in Beirut over the past year. Islamic Jihad (jihad means "holy war" in Arabic) has also declared that it is holding four of the six other westerners missing in Lebanon.

Yesterday, the badly decomposed body of one of those missing, a Dutch Jesuit priest, the Rev. Nicolas Kluiters, was found in a ravine in eastern Lebanon's Bekaa Valley. Dutch diplomats had said they believed theft of cash and checks Kluiters was carrying was the motive for his disappearance.

On Sunday, Danielle Perez, a secretary in the French Embassy in Beirut was released unharmed after being held for 10 days in west Beirut. Perez said she did not know who her captors were but a previously unknown group calling itself the Khaybar Brigades issued a statement last week saying it had abducted her, her father and another French diplomat. Perez said she and her father were kept in separate rooms.

A statement last week by the Khaybar Brigades had promised to release Perez and four others they said they were holding. Three of them, including Perez, have been released so far.

Police in Beirut told news agencies yesterday about a new wave of kidnapings of members of Lebanon's small Jewish community. They said gunmen seized a Jewish businessman, Ishaq Sassoun, on Sunday as he returned to Beirut from a trip to Abu Dhabi. He was the fourth Jew kidnaped in west Beirut in as many days.

Meanwhile, an airliner was hijacked and sectarian battles between Moslems and Christians erupted again in the southern Lebanese port city of Sidon.

The hijacker, described by Beirut radio stations as a member of one of the Shiite Moslem militia groups battling Israeli occupation forces in southern Lebanon, was armed with a pistol and a hand grenade.

He seized control of a Middle East Airlines passenger jet headed for Jeddah, Saudi Arabia, and demanded millions of dollars for one of the militia groups. After the control tower in Saudi Arabia refused permission for the plane to land, the hijacker surrendered peacefully.

The battles at Palestinian camps near Sidon continued for a sixth day of heavy fighting as an envoy representing Syrian President Hafez Assad came to Lebanon to urge President Amin Gemayel to increase the number of government soldiers in the southern port town, which was vacated recently by occupying Israeli soldiers in Israel's phased withdrawal from Lebanon.

U.S. sources have said that Syria has made efforts to free hostages held in areas of Lebanon it controls and, at its checkpoints, has posted photographs of some of the missing westerners. But these sources said Syria appeared to have refrained from forceful measures against the pro-Iranian militants that could strain its alliance with Iran.

Administration officials had blamed Syria and Iran for the attack on U.S. Marine headquarters in Beirut in October 1983 and had discussed retaliation then.

A few days after the attack on the Marines, Reagan had said: "Those who directed this atrocity must be dealt justice. They will be."

In the following three weeks, administration officials floated a variety of scenarios of the possible form of reprisals, but so far as is known, the United States did nothing.

French and Israeli warplanes did later attack garrisons of radical Shiite Moslem groups operating in Syrian-controlled eastern Lebanon who were believed to have played a role in the bombings, prompting questions at the time as to whether the United States was leaving it to other nations to do the retaliating.

Reagan, in the interview yesterday, said the United States had been "working as closely as we can" with allies to exchange information and deal with the problems of international terrorism and, "well, we have done something. I think we've had some measure of success . . .

"We know it's not just one group, there are a number of groups representing different interests. Sometimes they apparently collaborate and it's a new form of warfare."