A car bomb explosion followed by several blasts and rapid gunfire rocked the Afghan capital on Tuesday, not far from the heavily fortified Green Zone that houses several embassies, including the US mission.

The wave of blasts came as the Afghan army urged residents to evacuate a besieged southern city ahead of a planned offensive against Taliban insurgents after three days of heavy fighting.

The car bomb blew up in central Kabul late on Tuesday, sending a thick plume of smoke into the sky, AFP correspondents reported.

A security official said the explosion happened near Defence Minister Bismillah Mohammadi's residence, not far from the Green Zone.

"Do not worry, everything is fine," Mohammadi tweeted after the blast.

Less than two hours after the car bomb detonated, another loud blast followed by smaller explosions and rapid gunfire again shook Kabul, in what appeared to be the same area of the city.

Another security source said several attackers had stormed a lawmaker's house after setting off the car bomb and were also shooting at the residence of the defence minister from there.

"Several lawmakers were meeting at the house of this MP to make a plan to counter the Taliban offensive," the source told AFP.

A third Afghan security official said there "were fatalities" in the incident, but could not offer more details. 

Even as the blasts and gunfire rocked the city, crowds of people marched down Kabul's streets and took to rooftops chanting "Allahu Akbar" and "Death to the Taliban" in support of Afghan forces battling the Taliban in three regional capitals, witnesses said.

The insurgents' assaults on the cities of Lashkar Gah, Kandahar and Herat since last week came after they seized control of much of rural Afghanistan, as foreign forces began the last stage of their withdrawal from the country in May.

Taliban are everywhere
 

Fighting is raging for control of Lashkar Gah, the capital of southern Helmand province, with the United Nations saying at least 40 civilians were killed in the last 24 hours.

General Sami Sadat, commander of the 215 Maiwand Afghan Army Corps, urged residents to leave the city.

"Please leave as soon as possible so that we can start our operation," he said in a message to the city of 200,000 delivered via the media.

"I know it is very difficult for you to leave your houses -- it is hard for us too -- but if you are displaced for a few days, please forgive us.

"We are fighting the Taliban wherever they are. We will fight them... we will not leave a single Taliban alive," he said.

Officials said earlier that insurgents had seized more than a dozen local radio and TV stations in Lashkar Gah, leaving only one pro-Taliban channel broadcasting Islamic programming.

The UN Assistance Mission for Afghanistan (UNAMA) tweeted its "deepening concern for Afghan civilians... as fighting worsens".

"UN urges immediate end to fighting in urban areas."

Sefatullah, director of Sukon radio in Lashkar Gah said: "Fighting was intense this morning".

He said US and Afghan air force planes had pounded Taliban positions, and that fighting was ongoing near the city's prison and a compound housing the headquarters of police and intelligence agencies.

In recent days, the US military has intensified air strikes across the country in a bid to stem Taliban advances.

"The Taliban are everywhere in the city, you can see them on motorcycles in the streets. They are arresting or shooting people who have smartphones," a resident of Lashkar Gah told AFP, on condition of anonymity.

"The Taliban are in the people's houses and the government is bombing them. About 20 houses in my neighbourhood have been bombed, they are fighting street-to-street battles," he said.

War crimes
 

The loss of Lashkar Gah would be a massive strategic and psychological blow for the government, which has pledged to defend cities at all costs after losing much of the rural countryside to the Taliban over the summer.

In the western city of Herat that is also under siege, officials said government forces had managed to push back the insurgents from several areas -- including near the airport, which is vital for supplies.

But on Tuesday afternoon, four rockets struck the airport. The facility was not damaged, airport chief Shaheer Salehi told AFP, but two flights were cancelled.

Washington and London, meanwhile, accused the Taliban of committing atrocities that may amount to "war crimes" in the town of Spin Boldak, which the insurgents captured last month along the border with Pakistan.

Afghanistan's Independent Human Rights Commission earlier said the insurgents had carried out revenge killings there of at least 40 people.

"The Taliban chased and identified past and present government officials and killed these people who had no combat role in the conflict," the group said.