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 a university in Garissa. Troops are engaging the gunmen.
Witnesses said masked attackers fired indiscriminately and there are fears there could be more casualties.
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 the group told the BBC.
In previous attacks in north-eastern Kenya, al-Shabab spared the lives of Muslims while killing non-Muslims.
Live updates from Garissa siege
Exchange of fire
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.
A policeman at the scene told Reuters news agency that some students had been taken hostage.
"We can't tell how many but they are many since the college was in session," the unnamed policeman is quoted as saying.
The Kenyan Red Cross said about 50 students had been "safely freed", but an unknown number were still being held, AFP news agency reports.
Security forces have laid siege to the area around the university
Some students managed to flee the campus
Kenyans have repeatedly been targeted by al-Shabab
Security forces were now trying to flush out the gunmen, a police statement posted on Twitter said.
It urged people to stay away from the area. The statement did not confirm that hostages had been taken.
At the scene: Bashkas Jugsooda'ay, BBC Africa, Garissa
Fleeing students took shelter in nearby homes
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.
Most shops in Garissa are shut, and people are staying at home.
The town's hospital has been sealed off.
The gunmen reportedly ordered students to lie down on the floor, but some of them are known to have escaped and are at a military facility.
"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 in such a volatile area.
The Westgate attack in Nairobi in 2013 left 67 dead
The university opened in 2011 and is the only place of higher education in the region. It has some 700 students from across the country.
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.
Garissa, 150km (90 miles) from the border with Somalia, has a large population of Kenyan Somalis.
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.
The deadliest attack targeted the Westgate shopping centre in Nairobi in September 2013, when 67 people were killed.
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.
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