Gunmen from the militant Islamist group al-Shabab have killed at least 15 people and taken students hostage at a university in north-eastern Kenya.
Reports say 65 others were wounded when gunmen stormed the campus in Garissa. Troops are fighting the gunmen.
More than 500 students were still unaccounted for, a minister said. The number of hostages is unclear.
Al-Shabab, which is linked to al-Qaeda, said it was holding Christians hostage and freeing Muslims.
Hostages from the two groups had been separated, and 15 of the Muslims had already been released, a spokesman for al-Shabab told the BBC.
The Kenyan government has named Mohamed Kuno, a high-ranking al-Shabab official, as the mastermind of the attack, and placed a bounty of $53,000 (£36,000) on him.
‘Shot on the spot’
The gunmen reportedly ordered students at Garissa College University to lie down on the floor, but some of them escaped.
“It was horrible, there was shooting everywhere,” student Augustine Alanga told the BBC’s Newsday programme.
He said it was “pathetic” that the university was only guarded by two police officers.
Student Collins Wetangula said when the gunmen entered his hostel he could hear them opening doors and asking if the people who had hidden inside were Muslims or Christians, AP news agency reports.
“If you were a Christian you were shot on the spot. With each blast of the gun I thought I was going to die,” he said.
About five masked gunmen are said to have stormed the university.
Kenya’s Interior Minister Joseph Nkaissery told journalists that one of the militants had been killed as he tried to flee.
Out of 815 students, 535 had not yet been accounted for, he said.
It is not clear how many students in total were on the premises at the time of the attack.
The Kenya National Disaster Operation Centre said all staff at the university had been accounted for and were helping to track the students.
Kenyan officials say security forces have isolated the gunmen in a single building at the university. There were reports of some militants firing from a university hostel roof.
Security forces were now trying to flush out the gunmen, a police statement on Twitter said. It urged people to stay away from the area.
Kenyan President Uhuru Kenyatta offered his condolences to families of the victims in a statement.
He also ordered “urgent steps” to ensure police recruits could begin training immediately. “We have suffered unnecessarily due to shortage of security personnel,” he said.
At the scene: Bashkas Jugsooda’ay, BBC Africa, Garissa
I can hear gunfire from inside the campus. Ambulances are rushing in and out with the wounded.
One teacher told me some students managed to run away from the gunfire, and came to her house early in the morning to seek shelter.
But a huge crowd has gathered outside the house, mostly of people who are worried that friends and relatives may be still trapped inside.
Some of them are trying to enter the campus but the security forces are holding them back. Troops have also surrounded the main hospital, restricting public access to it as medical staff battle to cope with the wounded.
The university opened in 2011 and is the only place of higher education in the region.
The BBC’s Anne Soy in Nairobi says that because of its proximity to Somalia, Garissa is an easy target for al-Shabab militants and there have been several attacks in the past.
She says that the UK and Australia issued alerts this week warning of potential terror attacks in parts of the country, including Garissa.
There has also been a specific alert for universities in the country.
Recent al-Shabab attacks in Kenya
December 2014: Gunmen kill 36 quarry workers in Mandera after separating Muslims and non-Muslims
November 2014: Gunmen kill 28 non-Muslim passengers during an attack on a bus in Mandera
September 2013: Four gunmen take over the Westgate shopping mall in the Nairobi, killing 67 people
George Musamali, a security specialist and former officer in Kenya’s paramilitary police, told the BBC the authorities had “failed the students” by being poorly prepared.
“We’ve had intel [intelligence] for the last three months that al-Shabab were planning this kind of attack… and still they have been successful.”
Al-Shabab has carried out a number of attacks in Kenya since 2011, when Kenyan troops were sent to Somalia to help fight the militant group there.
Al-Shabab is fighting to create an Islamic state in Somalia and is banned as a terrorist group by both the US and the UK.