Facing ever more direct threats from the Reagan administration and increasing international isolation, Shiite Amal leaders showed no sign today of willingness to bend on the demand that more than 700 Lebanese detainees be freed by Israel before 40 Americans held hostage here are released.

There were indications that Syria had entered the picture as a potential mediator in the crisis. President Reagan reportedly has sent several messages to Syrian President Hafez Assad asking his assistance in winning the release of the Americans.

Reports said Nabih Berri, the Amal militia leader who has imposed himself as a middleman in the hijacking, would visit Damascus shortly.

Earlier in the week, Hojatoleslam Ali Akbar Hashemi Rafsanjani, speaker of the Iranian parliament, received the Shiite leader of Hezbollah, whose members are thought to be key figures among the hijackers. Rafsanjani then condemned the hijacking in strong language at a news conference yesterday, saying that if Iran "had known in advance about this kind of action, it would have acted to prevent it."

Diplomatic statements also focused on Israel today as United Nations Secretary General Javier Perez de Cuellar said the early release of more than 700 Arab prisoners held by Israel would be a "most essential and helpful step" in restoring peace in southern Lebanon. The statement contained no mention of the 40 American hostages held in Beirut.

In Bonn, Vice President George Bush responded to a question about the Arab prisoners in Israel by saying, "We think that people being held against international law should be released." At a press conference, Bush said the Reagan administration rejected linking the release of the Arab prisoners and the American hostages, but he seemed to emphasize the administration's "concern about prisoners being held in Israel."

The comments came as Israeli Prime Minister Shimon Peres confirmed in Jerusalem that he had sent a letter to Reagan urging maximum cooperation between the United States and Israel to resolve the crisis. Peres told reporters that "in this we stand by the United States."

Top officials of the International Committee of the Red Cross were reported to be visiting the hostages today. In a telephone interview with CBS, Berri said the Red Cross is checking on the American hostages "right now." [The Red Cross confirmed the report in New York.]

Amal officials refused to comment specifically tonight on press reports that the Reagan administration is considering measures to shut down the Beirut airport and apply economic pressure.

But neither was there any suggestion of new demands or retaliation against the hostages and senior Amal officials earlier in the day made it clear that the presence of U.S. naval forces in the waters near Lebanon would not be used as a pretext to keep the Americans captive if the central demand is met.

Berri, who also is justice minister of Lebanon's shaky central government, met with the British and Italian ambassadors. According to diplomatic sources, British Ambassador David Miers firmly warned him not to underestimate the anger of the Americans. Italian Ambassador Antonio Mancini, meanwhile, delivered a statement from the European Community condemning the hijack.

Some local reports also suggest that the European diplomats gave Berri an idea of the specific measures the Reagan administration may be contemplating. Berri told local reporters afterward that "the ambassadors' stand really tends to be inclined more towards a military than a negotiated solution."

While they condemned the TWA hijacking, moreover, "they did not pay attention" to the question of Israel's detention of men from southern Lebanon," Berri said.

A Syrian military officer has been closely monitoring the hijack crisis since last week, according to well-informed sources here, and Sheik Mohammed Hussein Fadlallah, the Hezbollah leader who visited Damascus, warned in a recent interview that if the crisis were to drag on, regional powers -- implying Syria -- would become involved. Syria has an estimated 30,000 troops in Lebanon.

Yet even as the pressure increases here, 12 days after the hijacking of TWA Flight 847, the Amal leaders appear to have little sense of the fury that it has provoked in the United States and much of the rest of the world.

Berri's deputy Akef Haidar asked that "a tiny problem, the hijacking of a plane," not be used to create even more suffering for this country that has been torn by 11 years of internal fighting and foreign invasion. "We've had enough of the civil war," the urbane, sad-eyed head of the Amal politburo told an afternoon press conference.

"We've lost a lot -- blood, money, and even civilization. It is not easy to regain what we have lost," Haidar said. "Really, we need to be helped by the big powers and especially by the United States of America."

"Instead of sending the fleet here let him President Reagan send some sympathy and he will gain more," Haidar said.

News services added:

*Israeli Foreign Minister Yitzhak Shamir was quoted by the state radio as saying Israel would support a U.S. blockade of Lebanon if diplomacy failed to free the hostages.

*Saudi Arabia issued a statement on the seizure of the TWA plane, saying the kingdom "denounces and condemns all forms of piracy and terrorism" as "counter to Arab and Islamic ethics and traits." The Saudis generally have refrained from commenting on Middle Eastern developments.

*In Madrid, two Lebanese Shiites whose freedom was once sought by hijackers of a TWA jet each were sentenced to prison terms of 23 years and four months for trying to assassinate a Libyan diplomat.

A Spanish court convicted Mohammed Rahal, 22, and Mustafa Jalil, 24, of illegal weapons possession and the attempted murder of Libyan diplomat Mohammed Idris on Sept. 12, 1984. said he would appeal.

Shiite Moslem militia leader Nabih Berri said last week in Beirut that the hijackers of the TWA jet had included the release of Rahal and Jalil among their initial demands. Berri has also said he persuaded the hijackers to drop that demand.