A group of Taliban gunmen attacked a school in Northern Pakistan on Tuesday, killing at least 145 people, most of them students under the age of 15. More than 180 others were injured. Police say the siege, which lasted about eight hours, is now over, with all of the terrorists dead. Other officials said four of the attackers blew themselves up in suicide attacks, though there may have been as many as nine attackers.

The Army Public School is a military-run primary school for the children of army members in Peshawar, in the northern part of the country, near the border with Afghanistan. Pakistani Taliban sources claimed responsibility for the attack, saying it was retaliation for an ongoing government offensive against them in the region of North Waziristan.

Witnesses says as many as six men scaled the fence of the school and began opening fire on students. Many students were reportedly killed by a suicide bomb in the school courtyard. The attackers then went room to room, shooting indiscriminately at children and teachers.

One witness told the BBC, "We ducked under the tables and chairs, but they shot at our heads and legs. The kept firing and coming further inside the room, but we did not move because they shot at anyone who moved."

The worst of the assault happened in an auditorium, where several classes had been gathered for first-aid training. One student who survived told Radio Free Europe, "It was 10:30 this morning when we were called to an auditorium to get first-aid training by an army colonel. When we walked in, gunshots erupted and [the militants] entered the auditorium. They killed many students —I saw about 40 to 50 students killed in front of me—and they also fired at the colonel."

According to the BBC World Service, the attack is believed to be an attempt to demoralize and humiliate the military. However, it may have the unintended effect of finally uniting the nation against the groups that have claimed tens of thousands of lives in Pakistan in the last 15 years. Today, both Prime Minister Nawaz Sharif and opposition leader Imran Khan issued strong statement condemning the attack, insisting the military offensive—which began in June with stated goal of driving the Taliban out of the region—will continue.

Sharif said today that “This is a decisive moment in the fight against terrorism. The people of Pakistan should unite in this fight. Our resolve will not be weakened by these attacks." This is the deadliest terrorist attack in Pakistan since 2007, when 140 people were killed in an assassination attempt on former Prime Minister Benazir Bhutto, who was later killed by another bomber.

World leaders elsewhere have condemned the attack as well. Indian Prime Minister Narendra Modi called it "unspeakable brutality." France's Francois Hollande called it "despicable" and the U.S. Ambassador to Pakistan, Richard Olson, says the country "stands in solidarity with the people of Pakistan, and all who fight the menace of terrorism."

In a statement released by the White House, President Obama said, "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."

Pakistan's Malala Yousafzai, who received the Nobel Peace Prize last week for her advocacy of education in Pakistan, said "I, along with millions of others around the world, mourn these children, my brothers and sisters—but we will never be defeated." Her co-winner, India's Kailash Satyarthi, told network NDTV that "I beg the Taliban, take me and leave these children."

This is a developing story and we will continue to update as more information becomes available.