Explosions targeting a high school in western Kabul on Tuesday have sparked widespread condemnation around the world.

Three explosions at a high school in western Kabul killed at least six people and injured 11, Afghan security and health officials said.

The blast targeted educational institutions and many of the casualties were young students.

"Three blasts have taken place...in a high school, there are some casualties to our Shia people," said Khalid Zadran, the spokesman for Kabul's commander.

He said later that six people had been killed and 11 injured in the explosions.

There was no immediate claim of responsibility for the attack.

An official familiar with the matter who declined to be named said the explosive devices had been hidden in backpacks and one had detonated inside the school gates, Reuters reported.

The incident was condemned both on a national and an international level.

Former president Hamid Karzai condemned the blast, calling it a crime against humanity and an act by the enemies of peace in Afghanistan and an act against education in the country.

The former chairman of the High Council for National Reconciliation, Abdullah Abdullah, also condemned the attack, calling those responsible the enemies of peace and development in Afghanistan.

The United States condemned the attacks and called for an investigation.

The US State Department in a statement offered condolences to the Afghan families and said the perpetrators must be brought to justice

"The United States joins the international community in expressing outrage at today's heinous attacks on the Mumtaz Education Center and the Abdul Rahim Shahid school in Kabul, Afghanistan. We offer sincere condolences to the families and other loved ones of those killed in these cowardly acts. The perpetrators must be brought to justice," US State Department Spokesperson Ned Price said in a statement.

"All Afghan children deserve to pursue their studies safely and without fear of violence," he added.

UN reacts to “heinous” attacks

The United Nations also condemned the deadly bomb blasts.

Ramiz Alakbarov, UN Humanitarian Coordinator in the country, issued a statement strongly condemning the “horrific” attacks.

“Violence in or around schools is never acceptable. For the people of Afghanistan, already beleaguered by forty years of war, schools should be safe havens, and places where children can learn and flourish,” he said.

Alakbarov underlined that attacks against civilians and civilian infrastructure, including schools, are strictly prohibited under international humanitarian law.

The UN Assistance Mission in Afghanistan (UNAMA) took to Twitter to condemn the “heinous” attacks.

“Those responsible for the crime targeting schools and children must be brought to justice,” the UNAMA tweet said, noting that Mission chief Deborah Lyons has extended her deepest sympathies to the families of the victims, and wishes for a speedy recovery for the wounded.

Filippo Grandi, the UN High Commissioner for Refugees, also joined Lyons in extending condolences to the bereaved families.

“Afghanistan’s ethnic, religious and linguistic diversity is at great risk. It must be respected and kept safe,” Grandi wrote on his official Twitter account.

UN Secretary-General António Guterres has also denounced the bombings.

“I condemn the deadly attacks on schools in Kabul and send my deepest condolences to the families of the victims. Attacks against civilians & civilian infrastructure, including schools, are strictly prohibited under international humanitarian law,” Guterres tweeted.

Save the Children organization in turn said: “We are horrified and outraged at the reported attack on a high school in Kabul.

“Children in Afghanistan have endured years of violence. Enough is enough. They have the right to access a safe education,” Save the Children said, added that: “The time is NOW to Stop the War on Children.”

Meanwhile, Amnesty International called the school bombing in Afghanistan a 'reprehensible attack' on religious and ethnic minorities.

Jasper Wieck, Special Representative of the German Federal Government for Afghanistan & Pakistan, U.S. Special Representative Thomas West and Chargé d’Affaires Ian McCary in series of tweets also strongly condemned the bombings.

“I strongly condemn today’s attacks in Kabul. Targeting the future of a country, its children & schools, is utmost heinous. It is the responsibility of the de facto authorities to protect the people of Afghanistan from violence & terror, no matter their age, gender & believes,” Wieck tweeted.

“The United States condemns in the strongest terms the horrific and cowardly attacks on students in Kabul today. No one should have to worry about their safety while being educated. My thoughts and prayers are with the families of the victims.” West tweeted.

“I deplore the horrific attacks on Mumtaz Education Center and the Abdul Rahim Shahid school in Kabul. We offer deepest condolences to the victims & families of the deceased. All Afghans should be able to pursue their studies safely and without fear,” McCary tweeted.

IEA says attack was “painful and sad”

IEAs officials meanwhile said they are committed to finding and punishing the perpetrators of the attack and preventing such incidents from taking place in future.

“The attack on our innocent children was very painful and sad. The Islamic Emirate is responsible for the security of every citizen,” said Inamullah Samangani, IEA’s deputy spokesman.