NAIROBI, Kenya (CNN) -- A pair of major explosions, presumably targeted at U.S. embassies, rocked two African capitals early Friday, killing at least a dozen people and injuring hundreds.

At least 10 people were confirmed dead in Nairobi, where an explosion ripped through the U.S. embassy and toppled a seven-story building next door in the city's crowded center about 10:30 a.m.

Hospital officials said more than 400 people were injured in the blast and made an urgent appeal for blood donations.

RELATED AUDIO

Local journalists report from the scene: Navraz Ahmad from dar es Salaam, Tanzania 352K/26 sec. AIFF or WAV sound Christine Nguku from Nairobi, Kenya 352K/26 sec. AIFF or WAV sound

An embassy official, who refused to identify himself, said he believed the blast was caused by a bomb.

"You can see a huge crater behind the building, and a bomb went off at the embassy in Tanzania at the same time," the U.S. official said.

The Associated Press reported that the Tanzanian blast, believed to have been a car bomb, came from the parking lot of the U.S. embassy in Dar es Salaam.

Officials told CNN it killed at least four people. Some reports indicated as many as nine people had been killed and 16 injured in the Tanzanian blast. Witnesses said it caused a large cloud of dust to rise over the capital.

Cranes were being rushed to the site to help lift debris and search for victims.

U.S. Marines immediately secured the U.S. compound in Dar es Salaam.

Reuters reported that the U.S. embassy in Uganda's capital, Kampala, was cordoned off and that police bomb experts were checking vehicles for bombs. Uganda borders both Tanzania and Kenya.

U.S. ambassador Prudence Bushnell was among those injured in the Kenyan explosion, officials said. Bushnell had just given a news conference at the Cooperative Bank. It's not clear if she was injured in the bank building or in the embassy.

The blast caused Nairobi's seven-story Ufundi Cooperative building, next door to the embassy, to collapse.

CNN reported that the force of the blast blew off the embassy's bomb-proof doors. The doors were used as stretchers to carry the injured away.

Hundreds of injured were sprawled screaming on the sidewalk with bloody wounds, and at least 54 were taken to hospital emergency rooms in Nairobi.

Two crowded buses and many vehicles were wrecked, witnesses said. The blast started many fires that sent a plume of black smoke into the sky, which was patrolled by police helicopters.

Shattered windows littered many downtown streets.

Ambulances and police rushed to the chaotic scene. Rescuers searched the rubble for survivors.

Armed U.S. Marines patrolled the streets outside both embassies.

A U.S. flag was blown off the embassy in Nairobi and was draped limply on an iron fence.

Correspondent Lara Logan, The Associated Press and Reuters contributed to this report.