The US embassy in Islamabad has tweeted the following:

These photos from the Reuters news agency show people injured in the attack being evacuated and treated by medical staff.

Image source, Reuters

Image source, Reuters

Image source, Reuters

Pakistani newspaper The Express Tribune is quoting officials as saying more than 30 people have been injured.

AFP news agency is quoting police as saying that the death toll has risen to 21, with most of the student victims shot dead inside a male hostel on campus. They add that security forces have ended the operation against the gunmen and are clearing the area.

The Pakistani military launched an operation against the gunmen, with a helicopter in support.

Image source, Reuters

Image source, Reuters

Image source, Reuters

The main challenge in the region where Charsadda is located is the rise in violence, especially that committed by the Taliban, says the BBC's Shazeb Jillani. 

The region is governed by the opposition party PTI, headed by the former cricketer Imran Khan. Though Mr Khan has long advocated for dialogue, he has been calling for tougher action in recent months, particularly after the 2014 Peshawar school attack.

Student Aizaz Khan was in the university when the attack began. He told Reuters news agency:

"We heard firing from the back of the campus. We thought maybe some people were fighting. Then the firing increased. Then we said: 'Get into the rooms. Don't go out'. Then the security forces came. They showed great bravery. The Pakistani army also showed great bravery."

Image source, Reuters

Rescue officials say they have recovered eight bodies. These are reported to include four security guards, a policeman, a chemistry professor and two female students.

Four gunmen are also said to have been killed.

Some reports suggest the number of casualties could be higher.

AFP news agency has quoted geology student Zahoor Ahmed as saying that he tried to leave his hostel after hearing shots fired. "(We) were stopped by our chemistry lecturer who advised us to go inside. He was holding a pistol in his hand," he said. 

Pakistan has been allowing teachers to carry weapons following the 2014 Peshawar school attack.

The BBC's Shahzeb Jillani says that the university had security guards posted at the main gate, but the attackers knew how to enter from the back, and took advantage of the low visibility caused by thick fog blanketing the area in the morning. Once spotted they began firing.

Dr Shakoor, an assistant professor with the university, turned back from the main gate of the campus as he arrived this morning after being told that the premises were under attack. 

He told the BBC's Ilyas Khan that many students were streaming out of that gate, apparently because attackers had entered the campus from the backyard where there is a university guesthouse.

The Pashtun nationalist movement from which Bacha Khan university draws its name were opposed to the Afghan Islamist resistance against the Russians during the 1980s and also oppose the Taliban, notes the BBC's Ilyas Khan. 

As such, an attack on the university holds symbolic value to Islamists.

The Bacha Khan University is a public university approximately 15km from the main city of Charsadda. Geo News reports, external that it is named after a Pashtun nationalist leader from the Partition era.

The gunmen opened fire on students and faculty members as they gathered at the school for a poetry recital to commemorate Mr Khan, Dawn reported, external.

Pakistani newspaper The Express Tribune is reporting that the Pakistani Taliban, also known as TTP, has claimed responsibility for the attack on the university.

Two distraught-looking men were seen escaping from the campus earlier.

Image source, Reuters / ARY News

The military says four gunmen have been killed.

Pakistani military and police have taken up positions around the perimeter of the campus.

Image source, AFP/Getty Images