A suicide bomber blew himself up in Afghanistan's capital of Kabul Tuesday, within 500 meters of the American embassy and reportedly targeting an American forces convoy, a source tells Fox News.

There were no reports of injuries or casualties, an Interior Ministry official tells Fox News.

An American Humvee was partially affected by the blast, according to witnesses.

The attacker was driving an explosives-packed vehicle, the Associated Press reported.

Police on the scene said casualties were expected. "It was a suicide car bomber, there are casualties but it is too early to know the extent of the damage," said Kabul deputy police chief Sayed Gulagha.

A spokesman for the NATO mission in Afghanistan, US Army Col. Brian Tribus, said that a coalition convoy had been attacked.

"We can confirm there was an attack on coalition forces. We are gathering information," he said.

The explosion happened at 1.20 p.m. on the main airport road in eastern Kabul. The blast sent a huge plume of black smoke over the city.

It happened as government employees were leaving their offices and roads were choked with vehicles as the working day is shortened during the Ramadan fasting month.

It comes a week after an audacious attack on the nation's parliament, which highlighted the ability of insurgents, who have been fighting to overthrow the Kabul government for almost 14 years, to enter the highly fortified capital to stage deadly attacks.

Also on Tuesday, a suicide attack on the police headquarters of southern Helmand province killed up to three people and wounded more than 50, including policemen, officials said.

Omar Zawak, spokesman for the governor of Helmand province, said most of the injured in the Tuesday morning attack were women and children.

Police spokesman Farid Hamad Obaid said a car packed with explosives was driven into the back wall of the police headquarters in an attempt to breach a gate. All the gunmen fled the area, he said.

Also Tuesday in eastern Paktya province, three people were killed and one wounded when their vehicle hit a roadside mine, the provincial police chief Zalmai Oryakhel said.

The Associated Press contributed to this report