King Hussein announced tonight that Jordan will send a contingent of volunteer troops to Iraq to fight alongside that country's forces in its war with Iran.

In the surprise announcement, delivered in an unscheduled television address, Hussein somberly said that the war, in which Iran apparently has taken the upper hand, has become a threat to the entire "Arab homeland," and he said he would join the Jordanian force when it goes to Iraq.

Hussein gave no indication of the size or the makeup of the force, but the assumption here is that it will include at least several thousand men, possibly a mixture of Jordanian Army troops and civilians with previous military training.

The decision was seen here as an indication of how badly the 16-month-old war is going against the Iraqi government of President Saddam Hussein and of the Jordanian king's determination to prevent Iran from expanding its influence in the region.

In Washington, the State Department said it had no immediate comment on Hussein's announcement. It was clear that the United States had been given no forewarning.

Jordan's entry into the Iranian-Iraqi war is expected to have major repercussions in inter-Arab relations, virtually ruling out any possibility of a pan-Arab consensus on the pressing issue of making peace with Israel.

Syria and Libya are helping Iran, and active Jordanian involvement on the side of Iraq could be one more divisive issue in an Arab world already badly divided on internal ideological issues as well as on the tactics of dealing with Israel.

Hussein indicated tonight that he expected other Arab states to send troops to assist Iraq, but he mentioned none by name. Informed observers said it was possible that some of the smaller Persian Gulf states such as Bahrain and Oman might send token forces.

Until now, Jordan has supported the Iraqi war effort only in nonmilitary ways, such as granting transit facilities for supplies through the Amman Airport and the Red Sea port of Aqaba and contributing food, medical supplies and construction materials.

The decision to send Jordanian troops to Iraq came just a week after Hussein visited that country as well as the Persian Gulf state of Bahrain, which has accused Iran of an attempt in December to overthrow its government.

Hussein characterized the war between Iran and Iraq, which began in September 1980, as a war that could eventually threaten "the gulf and the Arab homeland," and said it was being waged "against an Iranian enemy that is being supported by Israel and enemies of the Arab world."

He said he had "witnessed" the effects of the war in Iraq and, in visits to Iraqi soldiers, "found them firm and determined."

But he said he was "pained by a persistent question"--whether the Arab people were prepared to actively support Iraq.

"If you do not stand by Iraq now," he said he was asked, "will you later stand by the states of the gulf and the Arab Peninsula and the Arab world?"

In light of this broadened threat, Hussein asked, "Can our role be confined only to emotional participation and transport facilities? Is that all we can do?"

He said he would continue to press for negotiations to bring the war to a peaceful end, but in the meantime, "I hereby open before you the door to volunteer in the Arab Jordanian forces, the Yarmuk Force." Yarmuk was the site of a battlefield in northern Jordan where the Islamic armies won a decisive victory over the Byzantine forces in the late 7th century, opening the way for the spread of Islam throughout the region.

"I personally have the honor," Hussein said, "to announce that I volunteer in those forces as a Jordanian soldier to perform the most honorable and sacred duty.

"I hope to spend, God willing, every moment of my life in that arena where glory is made and Arab history is written anew, mixed with the blood of martyrdom."

Hussein said the decision to create the Jordanian force was justified under the principles of the Arab League charter and by "Arab resolutions on mutual defense."

Washington Post correspondent David B. Ottaway reported from Cairo that Jordan's move appeared to reflect an assessment that the Iranian-Iraqi war is coming to a showdown and that a last-ditch effort was needed to bolster Saddam Hussein's rule in Baghdad.

The Jordanian king, in his visit to Bahrain and other gulf states last week, attempted to stir up active support for Saddam Hussein and, at the same time, pledged to the smaller states that Jordan would defend them against Iranian expansionism.

The collapse of the Iraqi government would also strengthen the hand of Syria, Jordan's neighbor and rival to the north, further complicating Hussein's rule.

Although Iraq captured large swaths of Iran's border regions early in their war, the fighting bogged down for several months. Recently Iran, apparently having neutralized most of the domestic opposition to its revolutionary government, has begun to concentrate on the war again and its forces have inflicted heavy damage on Iraq's troops, according to reports reaching Washington.

Jordan's forces, while well-equipped and generally well-trained, have had little combat testing in recent years. In the 1973 Arab-Israeli war, Jordan belatedly sent an armored brigade of 80 tanks to the Golan Heights to assist Syria and it also sent officers and engineer units to Oman in the early 1970s to help that country put down a separatist war in its Dhofar Province.