Israeli forces warned yesterday that they intend to capture Beirut, the Lebanese capital, and they said they downed 25 more Syrian planes as U.S., Soviet and Arab leaders worked intensively to end the fighting and head off a new general Middle East war.

As wave after wave of Israeli warplanes bombed sections of Beirut and its southern approaches, other Israeli planes dropped leaflets urging Syrian troops to withdraw, stating, "In a short time we will capture the city." But Israeli ground forces and an armored column just outside southern Beirut appeared to be making little progress against stiff Palestinian and Syrian resistance.

U.S. efforts to persuade Israel to agree to a cease-fire and withdrawal--made partly at the urging of Soviet President Leonid Brezhnev and Saudi Arabian King Khalid--failed as Prime Minister Menachem Begin rejected two messages from President Reagan, the second stronger than the first, calling for a cease-fire and withdrawal from Lebanon.

U.S. dismay with Israel's stubborn position was made clear when Secretary of State Alexander M. Haig Jr. turned down an Israeli invitation to go to Jerusalem for talks, saying that Israel had not shown "sufficient flexibility to make a visit worthwhile at this time."

Israel, in responses termed "unsatisfactory" by one White House official, reportedly said it was agreeable to a cease-fire but would not withdraw its forces from Lebanon until it had assured the safety of its northern reaches from Palestinian shelling and attacks.

The reportedly strong U.S. messages and the criticism of Israel's position represented an overnight change from the previous four days, during which the Reagan administration carefully avoided recriminations against Israel while urging that it withdraw and all sides end the fighting.

Syrian President Hafez Assad, meanwhile, ordered his estimated 39,000 troops in Lebanon to "remain steadfast until victory," and Palestinian guerrilla leaders pledged to meet Israeli forces with house-to-house fighting in the Lebanese capital that they know so well.

Israel launched its invasion of Lebanon on Sunday, saying that it intended to clear Palestinian guerrillas from within 25 miles of the Israeli border. Yesterday, however, its northernmost forces were at Beirut's southern outskirts, 35 miles from the nearest Israeli border, and trying to push farther.

Israeli troop and armored columns, which had sliced and leapfrogged through the lower third of Lebanon with stunning speed in the first four days of the invasion, appeared to be stalled yesterday, however.

Lebanese, Syrian and Palestine Liberation Organization accounts all told of stiff resistance finally gaining a foothold.

Despite heavy Israeli bombing and shelling of Syrian and Palestine Liberation Organization positions near the Beirut International Airport on the capital's southern edge, PLO spokesman Bassam Abu Sharif said in Beirut that the combined forces repulsed six attempts by Israeli naval vessels to land tank reinforcements from the Mediterranean.

Israel's thrust up the strategic Bekaa Valley, a Syrian stronghold, and its attempt to cut the Beirut-to-Damascus highway also appeared to be stalled, with both sides holding day-old positions after heavy tank and artillery fighting.

A Syrian communique late last night said Syrian paratroops had landed in the southern Bekaa to help beat back an Israeli advance, Associated Press reported.

Once again, Israel's most spectacular successes appeared to be in the skies. Israel said its warplanes and artillery shot down 25 of Syria's Soviet-made Migs and three combat helicopters, with no Israeli losses.

By Israeli count, Syria has lost 61 warplanes in the five days of fighting--nearly one-seventh of its total combat force--while Israel has lost only one. Wednesday, in major air battles involving more than 150 planes, Israel said it shot down 29 Syrian war planes.

Syria said last night that it has lost seven planes and six helicopter gunships, as well as 83 tanks. It said it had destroyed 164 Israeli tanks. The Syrian communique also said 194 Syrian soldiers had been killed and 312 wounded and that Israeli forces suffered "very, very heavy losses." The PLO claimed yesterday that it shot down an Israeli plane over the Bekaa Valley.

Israel said yesterday that 52 of its troops have been killed so far in the invasion and 356 wounded. Later, the military command announced that Palestinian guerrillas in southeastern Lebanon had ambushed and killed Israeli Maj. Gen. Yekutiel Adam, 55, reportedly the highest ranking Israeli officer ever to die in battle.

The Palestinian Red Crescent put Arab casualties at 10,000, but it gave no breakdown of dead and wounded. An Israeli spokesman said yesterday that Israel had captured Palestinian guerrillas numbering in "three digits," but he gave no further details beyond saying that they would be considered prisoners of war and would not be harmed.

Damascus Radio accused Israeli warplanes of attacking a convoy of civilians near a Lebanese-Syrian border crossing, killing 57, but there was no confirmation. There also was no confirmation or further Syrian report on allegations by the Syrian news agency Wednesday--flatly denied by Israel--that Israeli planes had bombed near Damascus, killing eight persons.

Israel said yesterday's air raids into the Bekaa Valley were prompted by Syrian efforts to replace some of the SA6 antiaircraft missile batteries destroyed in two massive Israeli raids Wednesday. Israel had said it virtually wiped out Syria's 19 missile sites there, but some military sources here said there were indications some of Israel's claims on the numbers of Syrian planes shot down and missile sites destroyed have been exaggerated.

The dropping of thousands of leaflets on Beirut yesterday repeated an Israeli pattern carried out earlier in Tyre and Sidon, two coastal cities and former PLO strongholds. In the case of those cities, however, Israeli troops entered within hours after the leaflets were dropped.

The Beirut leaflets, addressed to the commander of Syrian troops in the capital city, said that Israel had committed "large forces" including "a huge number of tanks" to the capture of Beirut and contained maps with two suggested exit routes.

Reagan's series of urgent entreaties to Begin followed a message from Brezhnev containing what Haig called a "frank expression of concern" about the sudden expansion of the war Wednesday. Moscow has been Syria's chief non-Arab ally and its weapons supplier.

Saudi Arabia's foreign minister, Prince Saud Faisal, met with Reagan in Bonn to express Khalid's "serious concern" at the Israeli action, and Saud later told reporters that the Saudis expected the United States to give "a clear sign of its position" on Israel's "unprovoked and premeditated aggression."

U.S. special envoy Philip C. Habib remained in Damascus yesterday after going there from Jerusalem Wednesday in an effort to negotiate a cease-fire.

SANA, the official Syrian news agency, said the leaders of Kuwait, South Yemen and Algeria, in addition to Saudi Arabia, had called Assad to pledge support against Israel. Algerian President Benjadid Chadli reportedly sent a delegation to Damascus offering to help Syria.