At least 10 explosions - apparently from rockets launched by militants - and waves of automatic weapons fire were reported amid the drone of sirens and English-language warnings telling Americans inside the embassy to take cover.
It was unclear whether anyone - Western or Afghan - had been hurt or killed in the attack, but it appeared that one rocket had struck a minibus belonging to the Tak Beer private school, and witnesses said that young adults had been carried away bleeding and apparently unconscious.
Afghan officials said several attackers were behind the assault, but it was unclear precisely how many assailants there were or whether they were attacking from a single or multiple locations. The attackers were holed up on several floors of a tall, partially built concrete building that offered a bird's-eye view of the secured diplomatic and military compounds about a half mile away. Flashes from gun barrels could be seen as the militants fired from their perch. Afghan security forces returned fire from the ground, sending puffs of concrete dust into the air as bullets slapped the building.
"We don't know how many suicide bombers are in the building," said Col. Abdul Zahir, of the criminal investigative division of the Kabul police. "They're shooting at the embassy. We're still in fighting position. We can't say anything."
Two explosions were also reported near the Afghan Parliament, but it was unclear whether militants were specifically trying to attack the government building, or other targets.
The embassy assault, which began around 1:15 p.m., was the latest in a string of attacks that have chipped away at a tenuous sense of security in the capital. In August, militants killed eight people in an attack on a British cultural centre, and in June, nine suicide bombers breached layers of security to attack the hillside Intercontinental Hotel.
By 3:10 p.m., two Blackhawk helicopters circled the building, but did not immediately open fire.
The streets surrounding the site of the attack, normally choked with the traffic of minibuses, bicycles and Toyotas, were deserted on Monday afternoon of all but security forces and people racing for cover.
"We don't know what's happening," one Afghan soldier said. "Everywhere you can hear shooting."
Zabiullah Mujahid, a Taliban spokesman, claimed responsibility for the attack in a text message, saying the Taliban had set out to attack the embassy, a NATO base and Afghan government buildings. His claim could not be immediately confirmed.
The NATO-led International Security Assistance Force confirmed there were ongoing attacks against the embassy and ISAF headquarters, and said in a Twitter message there were "forces responding quickly," but provided no other details. The attack came less than two months after Afghan forces assumed formal responsibility for security in the capital, one of several corners of the country where security was officially handed over in July.
Sharifullah Sahak and Ray Rivera contributed reporting.