NAIROBI, Kenya -- Gunmen attacked a college campus in northeast Kenya early Thursday, opening fire in dormitories and killing at least 15 people and wounding 29 others, witnesses said.

The attack bore the hallmarks of Somali Islamic extremist group al-Shabaab, and there were reports that a spokesman for the group had claimed responsibility, but no official claims were posted on the websites and social media accounts used most frequently by the group.

"I am saddened to inform the Nation that early today, terrorists attacked Garissa University College killed and wounded several people and have taken others hostage," President Kenyatta Uhuru said in a speech to the nation.

He said details would be forthcoming "in due course" from security officials.

Augustine Alanga, a 21-year-old student who survived the attack at Garissa University College, described a panicked scene as gunshots rang out outside his dormitory in the pre-dawn hours when most people were still fast asleep.

The shooting became more intense almost immediately, he told The Associated Press by phone. The heavy gunfire forced some students to stay indoors as others fled with gunmen firing at them.

He said he saw at least five heavily armed, masked gunmen.

CBS News correspondent Debora Patta reported that students at Garissa college received letters about a week ago warning them of a possible attack, but it seemed no preventative measures were taken by security services.

A similar letter was sent to students at Nariobi University, warning of possible al-Shabaab attacks at major institutions in the capital, including the university. The letter simply advised students to be vigilant.

"I am just now recovering from the pain as I injured myself while trying to escape. I was running barefoot," said Alanga, who was one of scores of students who managed to escape through barb-wire fencing.

At the time the attack started -- 5:30 a.m. -- morning prayers were underway at the university mosque, where students were not attacked, he said.

A mortuary attendant in the town of Garissa says at least 15 people had been killed and at some 60 were injured. The attendant saw the casualties arrive by ambulance. He spoke on condition of anonymity because he was not authorized to speak to the press.

Some of the more serious wounded were being flown to Nairobi, the capital, authorities said.

The National Disaster Operations Center said on Twitter that three of four dorms were evacuated, with the gunmen cornered in one dorm. No further details were immediately available and Kenya Defense Forces have surrounded the area, journalists said, impeding their access.

Terrified students streamed out of buildings, some young men shirtless, as arriving police officers hunkered down, taking cover. The gunmen had opened fire at guards, triggering a "fierce shootout" with police guarding student dorms, Kenya's National Police Service said in a written statement.

Kenya's National Disaster Operations Center said 29 people wounded during the attack have been admitted to a local hospital, four of them in critical condition. Most have gunshot wounds, the center said.

Police and military surrounded the buildings and were trying to secure the area, police officer Musa Yego told AP.

Abass Gulett, head of the Red Cross in Kenya, said fighting was ongoing as the security forces try to retake some university blocks from the gunmen.

Al-Shabaab attacked the Westgate Mall in Nairobi in 2013, killing dozens.

In February, al-Shabaab released an online video in which it appeared to call for attacks on Western malls.

The video specifically named the Mall of America in Minnesota, the West Edmonton Mall in Alberta, Canada, and London's Oxford Street.

Kenya's northern and eastern regions, which are near the Somali border, have suffered many attacks blamed on the al Qaeda-linked Somali group, which has vowed retribution on Kenya for sending troops into Somalia to fight the militants. Kenya sent its troops there in 2011 to fight al-Shabaab militants following cross-border attacks.

Last month, al-Shabaab claimed responsibility for attacks in the county of Mandera on the Somali border in which twelve people died. Four of them died in an attack on the convoy of Mandera County Governor Ali Roba.

Police statistics show that 312 people have been killed in al-Shabaab attacks in Kenya from 2012 to 2014. Thirty-eight people were killed and 149 wounded in Garissa in the same period.