A young suicide bomber on foot penetrated the city's fortified diplomatic area Tuesday, leaving at least seven people dead and wounding more than 20 in the first blast to strike the Afghan capital's Green Zone since May 31, when a powerful truck bomb killed 150 people and wounded several hundred.

Police said all the victims were Afghan civilians, mostly people who worked in foreign embassies, Afghan government facilities and other offices inside the barricaded zone.

Afterward, pools of blood were scattered along the alley where the bomber detonated his suicide vest. Bystanders said those who died were probably walking home from work.

The Islamic State, through its Amaq News Agency, claimed responsibility for the blast. It followed a bloody streak of bombings and other attacks across the country by Taliban and Islamic State insurgents in the past several weeks that left more than 250 dead.

That surge came as the Trump administration prepared to add several thousand troops and step up its military role in the Afghan conflict. The Taliban has rebuffed recent official overtures to negotiate, demanding that all foreign troops leave the country first and declaring that it will press on with a campaign to show it cannot be defeated in battle.

The recent attacks have included bombings at several mosques in Kabul and other cities, the storming of an Afghan military base in southern Kandahar province, deadly attacks on security forces or government facilities in four other provinces, and a suicide attack on a bus carrying army medical cadets in Kabul.

After the May bombing, which spurred angry street protests and calls for President Ashraf Ghani's resignation, the government tightened security in the diplomatic area, adding new blast walls and barricades and posting additional armed guards. It also trained and armed civilian guards in the city's Shiite community after several recent bombings of Shiite mosques.

But the bombing Tuesday defied such measures and was expected to raise new concerns among city residents and international agencies. Officials said the target was not clear, but the site was close to the Canadian and British embassies as well as half a dozen Afghan government offices. Local employees there were sent into bunkers for several hours after the blast.

It was unclear how the bomber infiltrated the narrow street, which had high blast walls along one side and was heavily guarded at the entrance. Police said they were investigating. They described the attacker as an adolescent boy, which could have made guards less suspicious.

"I heard the blast and I rushed outside. There were at least six bodies lying in the street, and I helped gather them up," said Khalid Hussain, who works in the office of a national legislator a few yards from the blast site. "It was just after 4 p.m., so civilians were starting to leave their jobs and walk out."

Firetrucks, ambulances and other security vehicles came and went from the street for several hours after the blast, and helicopters circled overhead.

Long after dark fell, security forces lifted a temporary cordon and allowed hundreds of Afghan workers to leave, some pedaling bicycles down the alley and others hurrying anxiously on foot.

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