Two explosions struck within a dense crowd outside the Hamid Karzai International Airport in Kabul on Thursday, just hours after western governments had warned of a security threat there.

At least 12 American service members and scores of Afghan civilians were killed, officials said. The initial death toll ranged from 13 to 60.

Four US Marines were killed and three wounded, American sources initially told Reuters, in what the Pentagon said was a “complex attack” during its evacuation mission from Afghanistan.

Later, the American toll of 12 was confirmed by the head of the US Central Command.

The Islamic State claimed responsibility, the group’s Amaq News Agency said on its Telegram channel.

The night before the attacks, a senior US official had warned of a “specific” and “credible” threat at the airport by an affiliate of the Islamic State, the Islamic State Khorasan (linked to an old name for the region) or the ISIS-K, and western governments began urging people to leave the area.

Created six years ago by disaffected Pakistani Taliban, the ISIS-K is opposed by the US and the Taliban.

The Taliban spokesman, Zabihullah Mujahid, condemned the attacks and said that at least 13 civilians were killed and 60 wounded. Abdul Qahar Balkhi, a member of the Taliban’s cultural commission, said the attack was an act of terrorism, adding that the presence of foreign forces in the country was to blame.

US military officials at the airport said an attack, given the speed and confusion surrounding the entire evacuation, was never a matter of if, but when.

Even with a specific warning, the military officials said, it would be very difficult to pick out a suicide bomber with a concealed explosive vest in a huge throng of people, like that at the airport.

Since the Taliban takeover earlier this month, thousands of Afghan civilians and foreign citizens have gathered at the gates of the airport, which has a military and a civilian side, desperate to be airlifted out of the country. At times, the area has descended into chaos as people scrambled towards evacuation flights.

Two US military officials said evacuation flights were continuing, though it was not clear whether any gates at the airport were open.

“We can confirm that the explosion at the Abbey Gate was the result of a complex attack that resulted in a number of US and civilian casualties,” John Kirby, the Pentagon press secretary, tweeted. “We can also confirm at least one other explosion at or near the Baron Hotel, a short distance from the Abbey Gate.”

The Abbey Gate is a main entryway to the international airport.

The US Marines guarding the Abbey Gate had been briefed on the potential of a suicide bomber striking near their position, but continued processing those trying to gain entry.

One Afghan, Barat, who had travelled to the airport with his cousin to show documents to foreign soldiers, said he was about 30 feet away from one of the blasts.

“The crowd was packed and people were pushing,” he said. “I tripped — and that’s when the explosion happened. I think four or five soldiers were hit.”

Then chaos.

“We fell to the ground and the foreign soldiers started shooting,” Barat said. “There were bodies everywhere, people were running.”

Based on initial information, officials said, one thing appears clear: for all the years of bloodshed the Taliban caused in Afghanistan, they were not responsible for this new attack.

The Taliban, officials said, want to maintain an orderly evacuation at the airport, at least until the end of the month. Thursday’s explosions signalled that Afghanistan’s new leaders might not be up to the task.

The threats lay bare a complicated dynamic between the Taliban, al Qaida and the Haqqani network, and their bitter rival, the ISIS-K, in what analysts say portends a bloody struggle involving thousands of foreign fighters on both sides.

A UN report in June concluded that 8,000 to 10,000 fighters from Central Asia, the North Caucasus region of Russia, Pakistan and the Xinjiang region in western China had poured into Afghanistan in recent months. Most are associated with the Taliban or al Qaida, the report said, but others are allied with the ISIS-K.

“Afghanistan has now become the Las Vegas of the terrorists, of the radicals and of the extremists,” said Ali Mohammad Ali, a former Afghan security official. “People all over the world, radicals and extremists, are chanting, celebrating the Taliban victory. This is paving the way for other extremists to come to Afghanistan.”