KABUL, Afghanistan (CBS News) — The American University of Afghanistan was under attack, according to multiple accounts.

CBS News’ Ahmad Mukhtar reports from Afghanistan that several American professors are inside, along with possibly hundreds of students. Many appear to have escaped through emergency doors. The operation to clear the university is ongoing, and students reportedly remain trapped inside.

READ MORE: COVID: Vaccine Deadline Looms For San Jose Police; Mayor Says 85% Officers Vaccinated

Head of Kabul hospitals, Salim Rasouli, said they know that at least one guard was killed and 21 students, including four girls, were wounded and brought to hospitals, Mukhtar reports. This might change as the operation unfolds.

A U.S. official told CBS News at least one American was injured in the attack.

Witnesses say they heard gunshots, then a blast, and many believe it was the gate blown open, letting the attackers inside.

American University President Mark English confirmed an attack to the Associated Press but did not provide details.

Police spokesman Sediq Sediqqi said security forces were conducting a clearing operation to track down the “terrorists.”

Initial reports have two teams of attackers at the school, according to a U.S. law enforcement source, but the exact number of assailants isn’t known.

All other personnel on the campus were being evacuated, Sediqqi said. He had no further details on the nature of the attack.

The Pentagon said U.S. military advisers were on the ground with Afghan security forces at the university. Spokesman Adam Stump said the forces had been embedded with the Afghan units.

U.S. officials said the American forces embedded with Afghan units are in a non-combat, purely advisory role.

White House spokesman Josh Earnest said the Obama administration was deeply concerned about the attack.

State Department spokesperson Elizabeth Trudeau said the U.S. Embassy was working to account for all of its personnel and to locate and assist any U.S. citizens affected by the attack. She said American officials are monitoring the situation, but declined to provide details.

READ MORE: Concerns At Mineta San Jose International Airport After Man Drives Into Secure Area

“An attack on a university is an attack on the future of Afghanistan,” Trudeau said.

Pulitzer Prize-winning Associated Press photographer Massoud Hossaini had tweeted that he is trapped inside along with others, saying: “Help we are stuck inside AUAF and shooting flollowed by Explo this maybe my last tweets.”

His tweets about the incident were later deleted, and in an interview with his company, he said he was in a classroom with 15 students when he heard an explosion on the southern flank of the campus.

“I went to the window to see what was going on, and I saw a person in normal clothes outside. He shot at me and shattered the glass,” Hossaini said, adding that he fell on the glass and cut his hands.

The students then barricaded themselves into the classroom, pushing chairs and desks against the door, and staying on the floor.

Hossaini and about nine students managed to escape from the campus through a northern emergency gate.

“As we were running I saw someone lying on the ground face down, they looked like they had been shot in the back,” he said.

Hossaini and the nine students took refuge in a residential house near the campus.

Two professors — an American and an Australian — were kidnapped recently from the university on the 8th of August. Five gunmen wearing Afghan military uniforms abducted the pair at gunpoint.

The two foreigners were taken from their SUV while driving on Sunday night on a main road near the American University of Afghanistan, according to Sediqqi, the spokesman for the Afghan Interior Ministry.

Though kidnapping is sometimes the work of criminal gangs, the fear is that the hostages could be “sold up” to the Taliban, or the Islamic State of Iraq and Syria (ISIS), which has been fighting to gain hold in Afghanistan.

Kidnappings are not uncommon in Afghanistan. Three other foreigners who were kidnapped in Kabul over the past year have all been released, including an Indian woman, Judith D’Souza who was freed last month after being held for more than a month.

An Australian woman, Kerry Jane Wilson, was abducted in the eastern city of Jalalabad in April. Her whereabouts are unknown.

MORE NEWS: Sonoma Co. Wildlife Refuge Asks Public's Help To Rescue Cubs Orphaned By Caldor Fire

© 2016 CBS Interactive Inc. All Rights Reserved.