Terrorists strike again in Beirut.

The United States diplomatic presence in the Middle East received another major blow Thursday with the suicide car-bombing of the recently-opened American Embassy annex in east Beirut.

It was the fourth bombing of an official US facility in the Middle East in 17 months. According to early reports, at least 20 people were killed. Among the dozens injured were the American and British ambassadors who had been conferring in the building.

President Reagan telephoned US Ambassador Reginald Bartholomew to express his ''anger and sorrow.'' Commenting on the bombing before a speech at a campaign rally in Cedar Rapids, Iowa, the President said, ''We can't withdraw in the face of this kind of terrorism.''

The mysterious radical group ''Islamic Jihad,'' which claimed to have carried out last year's bombings of the US Embassy and Marine complex in Beirut and of the US Embassy in Kuwait, again claimed responsibility. A man who called foreign news agencies said Islamic Jihad would continue to strike at American targets ''until there is not a single American left in Lebanon.''

But the motive for this latest attack seems less clear than for the very similar attacks last year.

At that time the United States was deeply involved in Lebanese affairs. An 1, 800-man American Marine contingent was part of the multinational peace-keeping force intended to help stabilize Lebanon after the Israeli invasion. American diplomats were in the thick of negotiations over the country's future.

Today, there is no American military presence left to spark controversy. The main withdrawal took place in February, with the remaining 80 or so US combat troops pulling out at the end of July.

In addition, the United States abandoned its peace initiative in Lebanon in the spring. Since then, neighboring Syria had been the main force behind establishment of the new ''National Unity Government.''

The US, meanwhile, has maintained a very low profile over recent months. The one controversial move was the Reagan adminstration's Sept. 6 veto of a United Nations Security Council resolution calling for an end to Israeli restrictions on southern Lebanon. All 14 other members of the Security Council endorsed the resolution.

The US veto led to widespread condemnation in Lebanon, including from the government of Prime Minister Rashid Karami.

On Sept. 7 a man claiming to speak for Islamic Jihad telephoned foreign news agencies warning of an imminent attack in retaliation for the veto. Then, earlier this week a Beirut newspaper carried a small advertisement that read: ''US ambassador: wanted dead or alive.''

Despite the seeming power of Islamic Jihad, very little is known about the group. Most Lebanese and diplomats feel it is in fact not a single group at all, but a cover name or umbrella for several small cells of pro-Iranian Shiite Muslim fundamentalists who act independently.

The bombing, heard many miles away, seemed to underline the vulnerability of Western diplomats - particularly Americans. Although the Americans had greatly increased security precautions, turning their facilities in both halves of the capital into virtual fortresses, they were still unable to protect themselves.

American officials at the bomb site said that initial reports indicated an explosives-laden station wagon, resembling those driven by US security guards, was seen charging over the narrow mountain road in front of the six-story, modern annex shortly before noon.

American and British guards opened fire on the vehicle, which apparently prevented it from rolling into the front door and saved many lives, military sources said. British guards were outside the annex as British Ambassador David Miers met with US Ambassador Reginald Bartholomew. Both were slightly injured.

The bulk of the American diplomats in Lebanon had worked at the annex in Christian-controlled east Beirut since it was opened in July, even though the main embassy was in Muslim-dominated west Beirut. Attacks on US installations led to a shift of personnel to what was considered the safer, eastern, half of the capital.

There were early but unconfirmed reports that the station wagon had US diplomatic plates, explaining the reason it was able to penetrate multiple barricades of waist-high concrete blocks set along all entrances to the steep road of the Awkar suburb.

There have been at least a dozen US diplomatic vehicles stolen this year, all with the special white license plates carrying the number ''127'' signifying the US Embassy. Almost every diplomatic mission in Beirut, including the Soviet Union, have been victimized by unidentified gunmen who stop cars and vans, usually in broad daylight, to commandeer transport.

The bomb-laden vehicle would have had to go through Lebanese Army roadblocks to reach the US annex. This fact led to speculation here about just how much control the government or the Army has over Beirut, the only part of the country still under its jurisdiction, and what that bodes for the future.

The bombing comes at an especially awkward time for the Lebanese, as they struggle to prevent, through negotiations on constitutional reforms, a new outbreak of fighting. The latest security plan, implemented in Beirut over the summer, was also supposed to mean that the Army had removed the complex network of militias from the streets.

In the southern part of the country, which is prodominantly Shiite Muslim, resistance against Israeli troops has accounted for more than 150 deaths over the past two years.

(The Associated Press reports from Washington that US officials reacted Thursday with the following statements:

(President Reagan: ''We know that the worldwide terrorist movement has targeted a great many people, not only our own. We have been aware of this. You have to live and you have to protect yourself.... We can't crawl in a hole someplace and stop performing.''

(George Bush: ''The United States of America will never, ever permit terrorists to shape its foreign policy or influence our decisions one way or another....''

(Democratic candidate Walter F. Mondale: ''I join with all Americans in expressing my shock and outrage at this latest terrorist assault on our diplomatic personnel. I wish to extend my deepest sympathies to the families of those who have been killed or injured in this tragedy. If the perpetrators of this deed can be identified, I want President Reagan to know that he has my full support for appropriate countermeasures.'')

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