Afghan authorities investigated the attack on a heavily fortified government compound in the city of Kandahar that killed five envoys of the United Arab Emirates, the deadliest attack ever on the country’s diplomatic corps.

The five diplomats were among the more than 50 people killed in attacks on Tuesday in three Afghan cities, the Afghan Interior Ministry said. More than 100 others were also wounded in the bloodshed.

The Taliban, Afghanistan’s largest insurgency, struck twice with bombs near government office buildings in the Afghan capital Kabul, killing 38 people. It also hit an elite intelligence-agency squad in Helmand province, killing seven people.

The attacks were the deadliest in Afghanistan since November and again underlined the perilous security situation in the country, which has seen an increase in attacks since an international military force ended combat operations in 2014.

Neither the perpetrator nor the target of the bombing of the government guesthouse in Kandahar are known. At the time of the explosion, Jumaa al Kaabi, the U.A.E. ambassador to Afghanistan, was meeting Humayun Azizihad, the provincial governor. In all, 11 people killed in the explosion.

Mr. Kaabi was wounded in the blast and has been flown to the U.A.E. for medical treatment, Afghan Interior Ministry spokesman Sediq Sediqqi said.

Afghan security officials on Wednesday said investigators were looking at how the blast could have occurred despite tight security at the government compound, which is protected by dozens of armed guards and checkpoints.

They were also examining the possibility that explosives had been placed in the sofa where the Emirati diplomats sat, an Afghan security official said.

Before the envoys’ trip to Kandahar, Mr. Kaabi had expressed concerns to a Journal reporter about security, saying local authorities had failed to keep the timing and details of the diplomats’ visit confidential.

Haroon Chakhansuri, spokesman for Afghan President Ashraf Ghani, said the country’s national security adviser would lead the investigation into the Kandahar blast. He also said Mr. Ghani had spoken by telephone with Sheik Khalifa bin Zayed Al Nahyan, the U.A.E.’s president, and both had pledged to continue to work together.

“Such terrorist and barbaric attacks cannot harm our strong friendship,” Mr. Chakhansuri said.

The Emirati delegation, part of a new department at the U.A.E. embassy in Kabul to coordinate public and private-sector Emirati aid to Afghanistan, had traveled to Kandahar to open an orphanage and to launch a scholarship program.

The U.A.E.’s official WAM news agency named the five dead diplomats as Mohammed al-Bastaki,Abdullah al-Kaabi,Ahmed al-Mazroui,Ahmed al-Tunaiji and Abdul Hamid al-Hammadi. Mr. Kaabi wasn’t related to Jumaa al Kaabi.

Despite the large casualty toll, the attack wouldn’t change the U.A.E.’s aid strategy there, said Sheikh Mohammed bin Rashid al-Maktoum, the ruler of Dubai, WAM reported. A U.A.E. official said it could mean increased security for diplomats, however.

The U.A.E. was one of only three countries, along with Pakistan and Saudi Arabia, to recognize the Taliban government during its five-year rule of Afghanistan. Since the toppling of the Taliban in 2001, the U.A.E. has provided significant development aid to Afghanistan.

In 2015, the most recent year for which official figures are available, the country supplied $57.7 million in assistance, mostly for infrastructure development, humanitarian aid and transportation. Among a number of projects, the country paid $29 million to complete 40,000 housing units, according to an annual report by the U.A.E. Ministry of Foreign Affairs.

Among previous attacks on its diplomats, a gunman killed Khalifa al-Mubarak, the U.A.E.’s ambassador to France, in 1984, shooting him in the head with a pistol at close range.

Another gunman shot and wounded an Emirati diplomat later that year in Rome, an attack claimed by a Palestinian group angered by what it called the U.A.E.’s pro-U.S. and anti-Palestinian policies.

A car bombing carried out by jihadist group al-Shabaab in Mogadishu in 2015 targeted U.A.E. officials there, including the ambassador to Somalia, Mohammed al-Hammadi. He was unharmed.

On Wednesday, the Taliban released a video of two hostages, Australian and an American, who had been kidnapped in the Afghan capital in August. The two men were teachers at the American University of Kabul and seized from their vehicle at gunpoint on their way home from work.

The Navy SEALs conducted an unsuccessful operation to rescue them days after the kidnapping, the Pentagon said in September 2016.

The teachers, appearing frightened and disheveled in the 13-minute video, appealed to their families, the American government and President-elect Donald Trump to free Taliban fighters held at the high-security prison at Bagram in exchange for their release.

“We just ask that you please do this so that we can see our families again, so that we don’t have to die. That’s all we ask,” the American, Kevin King, said.

— Habib Khan Totakhil and Ehsanullah Amiri
contributed to this article.

Write to Jessica Donati at Jessica.Donati@wsj.com and Asa Fitch at asa.fitch@wsj.com