Last Updated Dec 16, 2014 3:49 PM EST
PESHAWAR, Pakistan -- Pakistani officials say Taliban gunmen stormed a military-run school in the northwestern city of Peshawar, killing at least 141 civilians and wounding many more.
Local officials tell CBS News that 132 of those killed were students, and nine were staff members.
Local hospitals report having a severe shortage of blood so more fatalities are expected.
The Pakistani military said in a statement late Tuesday that a rescue operation has been successful in ending the siege of the school, and that most of the students and the staff had been evacuated.
Pakistani Prime Minister Nawaz Sharif condemned the assault and rushed to Peshawar to show his support for the victims.
The horrific attack, carried out by a relatively small number of militants from the Tehreek-e-Taliban, a Pakistani militant group trying to overthrow the government, also sent dozens of wounded flooding into local hospitals as terrified parents searched for their children.
The number of terrorists killed in the operation remains unclear. Estimates from various officials range from 6 to 40 dead Taliban militants from the attack.
Explosives planted around the school were slowing the ongoing operation to clear the buildings, the military said.
In addition to the 148 reported killed so far, officials say another 121 were injured in the attack. Officials said 1,099 students and staff were registered at the school.
CBS News correspondent Clarissa Ward reports that Pakistan President Mamnoon Hussain has dubbed the incident a national tragedy. Most of those killed were believed to be between the ages of 12 and 16.
Officials said the majority of those killed were students, but hospital officials earlier said at least one of the fatalities was a teacher and one security official was also among the dead. The death toll was expected to rise.
The prime minister vowed that the country would not be cowed by the violence and that the military would continue with an aggressive operation launched in June in the North Waziristan tribal area to rout militants.
"The fight will continue. No one should have any doubt about it," Sharif said.
A student who escaped and a police official on the scene earlier said that at one point, about 200 students were being held hostage. Both spoke on condition of anonymity because they were not authorized to talk to media.
One of the wounded students, Abdullah Jamal, said that he was with a group of 8th, 9th and 10th graders who were getting first-aid instructions and training with a team of Pakistani army medics when the violence began for real. When the shooting started, Jamal, who was shot in the leg, said nobody knew what was going on in the first few seconds.
"Then I saw children falling down who were crying and screaming. I also fell down. I learned later that I have got a bullet," he said, speaking from his hospital bed.
Another student, Amir Mateen, said they locked the door from the inside when they heard the shooting but gunmen blasted through the door anyway and started shooting.
Police officer Javed Khan said the gunmen entered the school on Tuesday morning. He said army commandos quickly arrived at the scene and exchanged fire with the gunmen.
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Pakistani television showed soldiers surrounding the area and pushing people back.
Taliban spokesman Mohammed Khurasani claimed responsibility for the attack in a phone call to media.
"I can confirm that there were at least six gunmen who stormed the school," a Pakistani intelligence official in Islamabad, the Pakistani capital, told CBS News' Farhan Bokhari. "They were all Taliban militants."
Security sources told CBS News the militants entered the school grounds by jumping over a fence at the back of the compound.
Bokhari said the militants appeared to have targeted a portion of the school where children between the ages of 13 and 15 were taking exams. About 1,600 students are enrolled at the school, in grades 1 to 10. The school is located on the edge of a military zone in Peshawar, but the students are both the children of military personnel and civilians.
A teaching assistant who escaped the school said on local television that militants were going from room to room and "beating children." According to multiple reports, the Taliban militants had been ordered to shoot older students but allow the younger ones to go free.
"My son was in uniform in the morning. He is in a casket now," wailed one parent, Tahir Ali, as he came to the hospital to collect the body of his 14-year-old son Abdullah. "My son was my dream. My dream has been killed."
Peshawar has been the target of frequent militant attacks in the past, and Tuesday's siege was a powerful reminder of the Pakistani Taliban's threats for retaliation after the central government decided -- under intense pressure from Washington -- to attack the group's sanctuaries in the country's North Waziristan region, along the Afghan border, in June this year.
The attack was condemned quickly by foreign leaders, and Pakistan's Nobel Peace Prize laureate Malala Yousafzai, who was herself shot by Taliban militants and almost killed in 2012 as she rode home from her school on a bus with other students.
"I condemn these atrocious and cowardly acts and stand united with the government and armed forces of Pakistan whose efforts so far to address this horrific event are commendable," said Yousafzai in a statement. "I, along with millions of others around the world, mourn these children, my brothers and sisters -- but we will never be defeated."
In neighboring Afghanistan, President Ashraf Ghani issued a statement condemning the killing of innocent children as "un-Islamic and inhuman," and extended his condolences to the families of those affected in the attack.
U.S. Ambassador to Pakistan Richard Olson called the attack "heinous" and said the "United States strongly condemns senseless and inhumane attacks on innocent students and educators, and stands in solidarity with the people of Pakistan, and all who fight the menace of terrorism."
Olson added that "few have suffered more at the hands of terrorists and extremists than the people of Pakistan."
President Obama issued a separate statement, saying that "by targeting students and teachers in this heinous attack, terrorists have once again shown their depravity."
"We stand with the people of Pakistan, and reiterate the commitment of the United States to support the Government of Pakistan in its efforts to combat terrorism and extremism and to promote peace and stability in the region," Mr. Obama said in the written statement.
Speaking from London, U.S. Secretary of State John Kerry called the school's attackers "assassins" who had a "dark and almost medieval vision" and added he knew how hard it was to send children out in "today's world."