An American Marine was killed today and three others wounded as they were trying to defuse a piece of ordnance inside the grounds of the international airport one day after their return here as part of the multinational peace-keeping force.

The accident immediately raised concern here about the impact of the news in Washington, where the Reagan administration has gone to great lengths to play down the risks of sending Marines to this war-torn country.

The White House said that President Reagan reacted with "shock" and "sorrow" but that the casualties would not alter the U.S. commitment to keep the Marines in Lebanon.

[Senate Foreign Relations Committee Chairman Charles H. Percy (R-Ill.) signaled a disagreement with the administration on the use of the War Powers Resolution and said that the U.S. troops would have to be out of Lebanon within 90 days unless Congress approves an extension of their stay. But Percy later told an interviewer that Congress should not hesitate to extend the Marines' mission if it seemed advisable. Details on Page A21.]

U.S. Navy Lt. Cmdr. Mark Stull told reporters at the airport tonight that the four Marines, all engineers, were attempting to defuse a 155-mm shell, which he called an "ammunition bomb," when it went off.

First reports said the four had been injured by an American-made cluster bomb. Later accounts said it was an artillery shell containing the tiny cluster bombs that were used here by the Israelis during the war. The conflicting reports, which included an initial Pentagon announcement that the weapon was a mine, left it unclear tonight what had caused the explosion.

The Pentagon identified the dead Marine as Cpl. David L. Reagan, 21, of Chesapeake, Va.

Pfc. Leslie R. Morris, 19, of Pasadena, Calif. was reported in serious but stable condition. The two Marines with lighter wounds, both in good condition, were identified as Cpl. Anthony D. Moran, 21, of Macon, Ga., and Lance Cpl. George Washington, 19, of Elgin, Ill.

The blast marred a Lebanese celebration of the opening of traffic between the two sectors of the city, divided between warring Moslem and Christian factions for the past seven years, and of the reopening of the international airport to civilian traffic after nearly four months of closure because of the fighting.

The accident occurred in the late afternoon at the southern end of the airport, where the Marines have set up positions overlooking Israeli troops located 500 yards away.

Lt. Col. Lee DeLorme Jr., an Army spokesman assigned here from the secretary of defense's office, described the scene of the accident as a "tarmac area" adjacent to the runway. The airport was the scene of extremely heavy fighting between Palestinian guerrillas and Israelis for two months this summer during the Israeli siege of Beirut, which finally led to the evacuation of Palestinian fighters from the city. The Israelis pounded the area with bombs, rockets and artillery, while the Palestinians mined it to prevent an Israeli advance.

French engineers, who along with the Italian and U.S. troops formed the first peace-keeping force sent here in August to oversee the evacuation, helped the Lebanese find and defuse mines on the roads leading to the airport. But they did not cover the whole grounds, which extend over a vast area on the southern outskirts of Beirut.

The Israelis, who pulled out from the airport yesterday, also were believed to have removed mines from the location that they had occupied since late June. After the Israelis moved out, the U.S. forces took control of the area.

The Marines and the French and Italian troops returned to Beirut after the massacre of Palestinian refugees in the Shatila and Sabra camps Sept. 16 through 18.

Stull said the 1,200 Marines now ashore and billeted in empty airport buildings were "obviously sad" at the news of the accident.

"But the morale is the same, and the mission will continue," he said.

"There is a lot of ordnance in the area, as you know," he said, adding that an investigation has already begun to discover how the shell, or bomblet, exploded.

Meanwhile, all roads between the Christian eastern and the Moslem western sectors of Beirut today officially were opened to traffic for the first time since the 1975-76 Lebanese civil war.

At a ceremony held at what was long known as the Museum Crossing between the two sides, President Amin Gemayel proclaimed the capital one city again in the presence of the international peace-keeping force.

"I am very happy to announce today that Beirut has again become the capital of all Lebanon," Gemayel said. "There is no more an East Beirut and a West Beirut. As of today, the whole capital will be reunited and this is not only the symbol of the reunification of the country but of our hearts."

Because of the evident concern about the president's security in the wake of the assassination of his brother, Bashir, the public was not present at the ceremony. In fact, the entire area around the national museum was damaged badly during the fighting this summer and most buildings stand gutted and empty.

Immediately after the ceremony, Gemayel attended the airport reopening. He watched the first Middle East Airlines plane in four months arrive and depart to the applause of a small crowd of Lebanese on hand for the event.

The city was transfixed by the welcome noise of commercial jets flying once again over the city, which for three months heard nothing but the screech of Israeli jets followed by devastating rocket and bomb explosions.

Outgoing Prime Minister Shafiq Wazzan marked the reopening of roads by officially declaring open the four-lane, elevated Fuad Chehab Highway connecting the eastern sector and the west.

French units of the peace-keeping force also began for the first time to patrol the streets of East Beirut. An Army spokesman said they would do this because there was only one Beirut now, and the French were responsible for providing protection in all of it.

But there were still remnants of the old block-by-block security system apparent today in the Lebanese Army checkpoints that have gone up throughout the capital to check cars and personal papers.

Other than the blast today, the landing of U.S. troops and their heavy equipment continued without incident.

The U.S. forces unloaded amphibious vehicles from landing craft on the beach across the main coastal road from the airport. After the sands were swept for mines, the first landing craft arrived, and roughly 70 Marines waded through light surf to the shore. Several of the first trucks off the craft immediately became mired in the soft beach sand.

Col. James Mead, commander of the Marines, said he had been somewhat worried about a noncombat landing because the Marines had no chance to practice. Despite the difficulty with the trucks, Mead said "it looks good."