"USA citizens in Kabul are advised to avoid traveling to that area", a statement by the embassy said. Eleven U.S. contractors were also hurt, though the injuries don't appear to be life-threatening, reports NBC News.
The bombing in Kabul's diplomatic quarter killed an Afghan guard at the German embassy and wounded two embassy staff, one of them German, the other one Afghan. The U.S. embassy is located about half a mile from the scene. Numerous victims appear to be civilians, including women and children.
No group immediately claimed responsibility for the blast.
Yarmand said he didn't think the Taliban were behind it, saying they "don't have the ability to carry out such big attack, and if they did, they would have claimed responsibility".
AFP PHOTO / WAKIL KOHSARAn Afghan resident stands alongside a man wounded in a vehicle bomb attack at a hospital in Kabul on 31 May, 2017. They're now one of the biggest contributors to the NATO-led Resolute Support mission with around 980 soldiers on the ground to support and train Afghan security forces. The Islamic State has said it was responsible for other recent bombings in Kabul, including one that killed eight people and injured 22 earlier this month.
Just hours after Germany's foreign minister announced all of its embassy staffers were safe, the country's interior minister confirmed that it would only briefly suspend its policy of deporting rejected Afghan asylum seekers to Kabul out of "respect for its Embassy staff".
The U.S. Embassy urged all U.S. citizens in Afghanistan to review their personal security plans, take appropriate steps to enhance your personal safety, remain aware of your surroundings, monitor local news for updates, and maintain a high level of vigilance.
"There are a large number of casualties, but I don't know how many people are killed or wounded", said an eyewitness, Gul Rahim.
"I felt like it was an natural disaster, and after that I do not know what happened", said Mohammed Hassan, 21, who was attending a training program at the Azizi Bank, half a block from the blast, and suffered cuts on his head and arms. In Paris, Marielle de Sarnez, France's Minister of European Affairs, said her country's embassy was among those damaged.
The blast in the Wazir Akbar Khan area happened near several embassies and not far from the presidential palace.
The explosion occurred near Zanbaq Square close to the embassies of Germany, Turkey and Japan and in front of the National Directorate of Security (NDS) compound.
BBC Bureau Manager Karim Haidari posted a picture on twitter of the colleague who died in the incident.
It is believed to be either a truck or tanker full with explosives went into diplomatic neighborhood which is highly secured and guarded. If so, that would constitute a massive security breach.
The Islamic State (ISIS/ISIL), considered a rival by the Taliban, is known to operate in Afghanistan and launch attacks against civilians in Kabul, particularly during Ramadan.
President Ashraf Ghani condemned the attack. The blast left a gaping crater, at least five meters (yards) deep.
Secretary of State Rex Tillerson condemned the attack, offering condolences to the families and friends of the victims and prayers for recovery of those wounded.
"The vigilance and courage of Afghan security forces prevented the VBIED (vehicle-borne improvised explosive device) from gaining entry to the Green Zone, but the explosion caused civilian casualties", North Atlantic Treaty Organisation said in a statement.