KENNEBUNKPORT, MAINE, AUG. 22 -- President Bush today ordered the first mobilization of U.S. military reserves in 20 years and declared the call-up "essential to completing our mission" of thwarting Iraqi aggression in the Persian Gulf.

In a news conference at his summer home here, the president repeatedly sought to reassure Americans that "we are not in this alone." Despite Iraqi President Saddam Hussein's effort to portray the gulf crisis as a confrontation between his country and the United States, "it is between Iraq and the entire world community," Bush said.

Neither Bush nor Defense Secretary Richard B. Cheney, who briefed the president this morning, would disclose how many reservists will be activated by the call-up order, but administration officials indicated that about 40,000 troops will be summoned initially, with more likely in the future.

As Bush spoke here, Jordan's King Hussein told a news conference in Amman that he will visit Baghdad and other Arab capitals as part of a diplomatic peace initiative {Details on Page A35}. Meanwhile, amid the turmoil in the Middle East, crude oil prices jumped more than $2.50 a barrel, breaking the $31-a-barrel level for the first time in almost five years.

Cheney is scheduled to meet with the military service secretaries and other Pentagon officials Thursday to review their requests for reserve forces. Bush has the authority to activate up to 200,000 reservists, but Cheney said the number would fall well short of that.

A substantial majority of those summoned to active duty in the initial mobilization likely will be Army reservists, Pentagon officials indicated. Navy sources said their service anticipates a need for 5,000 to 8,000 reservists, while an Air Force colonel said that "right now we don't have any need for the reserves."

An Army colonel said the call-up from that service probably will total "somewhat less than 40,000" but "there will be more in the future, probably in 30-day increments." Army officials tonight were drafting "alert messages" notifying specific reserve and National Guard units that they may be activated; the actual call-up will be issued in a separate order. Of the three major services, the Army is most dependent on reservists in a crisis to fill the ranks, particularly in service and support jobs; half of the total Army is made up of reservists and National Guard personnel.

Senate Armed Services Committee Chairman Sam Nunn (D-Ga.) applauded the president's order.

"It is no secret that reserve and Guard units contain skills and perform functions that are not available in the active forces," Nunn said. Senate Majority Leader George J. Mitchell Jr. (D-Maine) also continues to support the administration's actions, an aide said.

Pentagon officials said that most reservists probably will be used to fill vacancies in the United States left by active-duty military personnel being shipped to the Mideast. Army officials said they needed skilled maintenance and medical reservists, including port workers, water purification teams, doctors and nurses. A Navy officer cited a need for combat search and rescue units, cargo handlers, doctors, and shipping control and port security personnel.

As the U.S. forces in Saudi Arabia gather a greater offensive capability, the demands for more support troops -- from ammunition handlers to medics -- will also grow, a Pentagon official said. Eventually the ratio of support personnel to combat troops could reach 3 to 1. That will require greater reliance on reservists, who account for 70 percent of the Army's support capabilities.

The harsh demands of sustaining -- and potentially fighting -- in the desert are shaping the decisions about reserves. Alluding to the failed attempt in 1980 to rescue Americans held hostage in Iran, an Army colonel said, "Desert One is driving a lot of this."

After meeting with Cheney, Gen. Colin L. Powell, chairman of the Joint Chiefs of Staff, and others to review the military situation in the gulf region, Bush told reporters he is willing to give the United Nations more time to approve a resolution authorizing use of force to implement the U.N. economic sanctions against Iraq.

But he said the United States has "all the authority we need" to act unilaterally to enforce the sanctions militarily if the U.N. effort falters.

U.S. officials continued their behind-the-scenes diplomacy in an effort to win support for the U.N. resolution from the Soviets, who have been balking on the grounds that it is premature to endorse military force until it is clear that the sanctions are not being enforced.

U.S. reservists were last activated for military duty in January 1968 after the Tet offensive in Vietnam, when President Lyndon B. Johnson called up 35,280 soldiers. Nixon also called up reservists in 1970 during a nationwide postal strike.

The president said he was encouraged by the report he had received from Cheney and Powell about the deployment of the equivalent of a medium-sized American city to the Persian Gulf in the last two weeks.

Powell declined to reveal how many American troops are now on the ground there, but said the United States is not trying to match "man for man" the roughly 160,000-strong Iraqi army on the border with Saudi Arabia.

With each passing day, Powell said, "I am more and more confident of our ability to accomplish" the mission set out for the troops.

Cheney, who returned Tuesday from the Middle East, said that in an effort to enhance Saudi defense capabilities, the United States would sell the Saudis a number of F-15C and D fighter jets from U.S. stocks. He said the Saudis would not receive the F-15E, the most advanced version of the plane. The F-15E is capable of carrying bombs to destroy ground targets as well as fighting in air-to-air combat against enemy planes. An Air Force spokesman said the United States has 460 of the less-sophisticated fighters in inventory and 200 of the F-15Es.

The president also said the United States and other countries would not bow to Iraqi demands that all embassies in Kuwait be closed on Friday. "We must not take the position that this illegal regime can shut down legitimate embassies as a result of their aggression," he said.

The United States is attempting to draw down diplomatic personnel to a minimum at the embassy in Kuwait, officials said. The Iraqis have warned that as of Friday, any embassy officials still remaining will lose their diplomatic status and will be treated like other U.S. citizens now held hostage there.

Bush was asked whether he felt constrained in his decision-making by the danger that the Americans and other foreign nationals would face if military hostilities erupt. He replied that while he would weigh the lives of innocent people carefully, the U.N. sanctions "must be enforced."

As Bush raised the ante on U.S. military involvement in the Middle East by activating reserve forces, he stressed the participation of many other nations in the conflict. "We stand shoulder to shoulder right there in the Middle East with the armed forces of 22 other nations from the Middle East, from Europe and around the world," he said.

He also continued to rally support for the unconditional withdrawal of Iraqi troops and the restoration of the Kuwaiti government. "The world simply cannot waver in its opposition to the threat that Iraq has placed on the doorstep of all nations who cherish freedom and the rule of law," the president said.

With Jordan's King Hussein planning another visit to Baghdad, Bush said he had not asked the king to take a message to Saddam "other than one: our determination to stay joined up with others to see that this aggression is reversed and that the rightful rulers of Kuwait are returned."

He said Saddam's growing isolation from Arab and other nations had created a sense of "irrational urgency" in Baghdad, but made clear the United States won't begin to negotiate with the Iraqis until they abandon conditions he called unacceptable under international law.

After firing warning shots at two Iraqi tankers on Saturday, the United States has eased off militarily for the time being in an effort to win support at the United Nations for military enforcement of the sanctions. U.S. officials said today they had succeeded in getting the approval of China, one of five permanent members of the Security Council, with the Soviet Union now the only clear holdout.

U.S. officials said they believed they have more time to press the U.N. negotiations because there appears to be no imminent likelihood that the Iraqi tanker being shadowed by naval forces in the gulf will deliver its cargo of oil. Bush said the United States preferred unanimity on the issue to send a signal to Saddam that many nations are prepared to back up the sanctions, but that he remains prepared to intercept the ships and that there are adequate forces to do so.

In other military developments, the aircraft carrier USS Saratoga moved through the Suez Canal today, giving the Navy three carriers in waters near the gulf. The carrier USS John F. Kennedy, which left Norfolk last week, is still in the western Atlantic, where its air crews are practicing. The Kennedy is expected to proceed to the eastern Mediterranean soon, giving the United States four carrier groups with approximately 36,000 sailors and pilots in the vicinity. The Kennedy's arrival will bring to more than 60 the number of warships near the Arabian peninsula, a Navy officer said.

Balz reported from Kennebunkport and Atkinson from Washington. Staff writer Gwen Ifill contributed to this report from Washington.