Nairobi citizens and rescue teams cut through the ruins of the Ufundi Cooperative building

NAIROBI, Kenya (CNN) -- Rescue workers used backhoes and bare hands Saturday to free people trapped under mangled steel and concrete shattered in car bombings at the U.S. embassies in Kenya and Tanzania.

At least 139 people, including 11 Americans, died in the blast, which heavily damaged the embassy building and incinerated two passing buses. Scores of people were missing, and more than 1,200 were injured.

In neighboring Tanzania, an almost simultaneous blast at the American embassy in Dar es Salaam killed at least nine people and injured 57. No Americans were reported killed there, but the embassy building, in a residential area of the capital, was heavily damaged.

The U.S. State Department reported that six Americans were still missing in Nairobi and 14 were hospitalized. U.S. Ambassador Prudence Bushnell was slightly injured.

The Americans were believed to be part of the U.S. diplomatic delegation in Kenya, not private citizens visiting the country.

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As dawn broke in Nairobi Saturday, rescuers could hear at least three people calling out from inside a debris-choked elevator shaft, plus a fourth person trapped nearby.

"It is horrific to hear moaning, sighs and whimpers from under the rubble when we can do so little to reach victims quickly," Red Cross spokeswoman Nina Galbe said.

Working through the night beneath floodlights, rescuers in both capitals used pick axes, shovels and ropes to clear rubble.

After an hour-long effort in Nairobi, one person was pulled alive from the wreckage to cheers from the gathered crowd. But another rescue attempt ended tragically.

Kenyan Sgt. David Kambi, an army engineer, said he struggled for four hours to free a 40-year-old man named Gitau whose chest was weighed down by a slab of concrete.

"I told him, 'Gitau we're going to help you,' " Kambi said. He pleaded for help twice, then died, the officer said.

U.S. officials say that the attacks, which came without warning, appear to be a coordinated terrorist assault, and they are vowing to track down and punish those responsible.

"We will be relentless and persevering," said U.S. Undersecretary of State Thomas Pickering. "[The investigation] will involve all the forces we can bring to bear on it."

U.S. diplomatic outposts around the world were put on heightened alert.

No one has claimed responsibility for the blasts. While U.S. officials are declining to say who they think might be responsible, they have indicated that the parties are likely from outside of the two East African countries, with which the United States has generally good relations.

While Kenya has been experiencing some internal strife in recent years, political violence is virtually unheard of in Tanzania, among the most stable of African countries.

Police in Nairobi were seen taking an Arabic-speaking man into custody, but authorities would not comment on his connection, if any, to the blast.

An Egyptian militant group, Islamic Jihad, a successor to the group that assassinated Egyptian President Anwar Sadat in 1981, vowed last week to strike American interests after some of its members were arrested in Albania and handed over to Egypt, according to a report Thursday in Al-Hayat, an Arabic-language newspaper in London.

The Egyptian Islamic Jihad group is believed to have links to Osama bin Laden, a Saudi dissident living in Afghanistan, who the U.S. State Department considers the prime suspect in the 1995 car bombing in Riyadh, Saudi Arabia, that killed five Americans and the June 1996 blast at a military housing complex near Dhahran, Saudi Arabia, that killed 19 U.S. servicemen.

U.S. President Bill Clinton denounced the attacks as "abhorrent" and "inhuman," and he promised to bring the perpetrators to justice "matter what or how long it takes." He planned to devote his weekly radio address Saturday to the U.S. response to the attacks.

U.S. Secretary of State Madeleine Albright, who was on a personal visit to Rome, planned to return to Washington Saturday morning to meet with a task force set up to investigate the blasts.

The United States dispatched disaster response teams and FBI investigators from both the United States and a U.S. base in Germany.

Pickering said American teams from the Middle East also will provide additional security at the Kenyan and Tanzanian embassies.

Israel dispatched search-and-rescue teams, experienced in evacuating people from bombed buildings, to Nairobi to assist in the rescue effort.

Smoke rises from the U.S. Embassy in Dar es Salaam, Tanzania

The blast in Nairobi occurred about 10:45 a.m. (3:45 a.m. EST/0745 GMT). The force of the blast blew off the embassy's bomb-proof doors -- which were later used as stretchers to carry away the injured.

Injured people were rushed from the scene, as a plume of smoke rose above the Nairobi skyline. Windows were shattered as far as 10 blocks away, and bloodied clothing and papers littered the streets.

Crowds crawled over the twisted and broken concrete and metal that was once Ufundi House, looking for victims and trying to free trapped people heard crying for help. As darkness fell, studio lights were set up so that the rescue work could continue.

"We fear the worst. By the time the rubble is cleared, we expect to find more dead," said Red Cross spokeswoman Nina Galbe. The city's four hospitals were overwhelmed with the injured.

The Kenyan government announced an official five-day mourning period for victims of the bombing and ordered flags lowered to half staff. The government pledged $850,000 to a fund to help survivors and families of the dead.

President Daniel arap Moi toured the scene and visited some of the injured. He issued a statement vowing to do everything possible "to bring the perpetrators of the heinous crime to book."

According to the State Department, the blast in Dar Es Salaam occurred about five minutes before the one in Nairobi. However, some reports had the order of the blasts reversed.

A corner of the embassy building collapsed onto an area used to park cars, and some embassy personnel were trapped under rubble inside the embassy. A guard shack was destroyed, and nearby streets were covered with debris, including the mangled, blacked wreckage of a car directly in front of the building.

"About 20 or 30 meters around this place looks like a war-torn area," said Navroz Ahman, a witness. "The houses have been blown to pieces.... The rooms have been blown off."

The nearby French and German embassies were damaged, but no one in either building was hurt.

U.S. Marines rushed to evacuate the building, then cordoned off the area with pistols in hand. As night fell, the embassy, set near the Indian Ocean about two miles (3.2 km) north of the city center, was deserted, except for the Marines and a contingent of Tanzanian riot police.