A powerful blast followed by sporadic gunfire hit Afghanistan's capital Kabul today near the city's heavily fortified 'Green Zone', an area home to government buildings and foreign embassies, police officials said.

At least three people were killed and seven wounded, said Ghulam Dastagir Nazari, a health ministry spokesperson, adding that clashes were continuing.

A senior security official said the blast appeared to have been caused by a car bomb and the apparent target was the residence of a member of parliament.

Two gunmen were still in the area and clashing with Afghan security forces, the official said. Several homes belonging to Afghan officials, politicians and prominent residents were under siege in the continuing clashes, residents said.

The city's Emergency Hospital said in a tweet it had so far received six people wounded in the attack.

No group immediately claimed responsibility.

Kabul bomb blast

Minutes after the blast, hundreds of civilians in Kabul came out on to the streets and chanted Allahu Akbar (God is Greatest) to express their support for Afghan government forces and opposition to the Taliban.

The overnight march spilled across the city with mostly men and some women joining in the demonstrations, carrying candles and Afghan flags to signal united opposition to the hardline Islamist group.

"The whole world can choose to be silent about what is going on in Afghanistan but we can't and won't stay quiet anymore...we will stand side by side with our security forces until our last breath," said a demonstrator in Kabul on condition of anonymity.

The United States condemned the blasts, suggesting it was likely they were carried out by Taliban insurgents.

"We're not in a position to attribute it officially just yet but of course it does bear all the hallmarks of the spate of Taliban attacks that we have seen in recent weeks," State Department spokesman Ned Price told reporters.

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Last week residents in western Herat braved the streets despite nearby fighting to protest against the Taliban. Other cities quickly organised to join the chant from their homes in the evenings, as a message of support for embattled security forces.

Clashes between Afghan forces and the Taliban have intensified across the country with the insurgent group gaining control over check points, trading posts and infrastructure projects.

Afghan forces appealed to residents of the southern city of Lashkargah to leave their homes and stay away from areas where the Taliban were taking control, as they intend to launch operations against the group.

The Taliban said their fighters in Kabul killed a district governor of central Maidan Wardak province today, the latest in a series of killings by the insurgent group aimed at eliminating senior government officials and social activists.

An Afghan National Army commando stands guard in Herat

Afghan army urges civilians to leave besieged city

The Taliban has seized control of much of rural Afghanistan since foreign forces began the last stage of their withdrawal in early May.

The insurgents are now focused on capturing provincial capitals, where they are meeting stiffer resistance.

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

General Sami Sadat, commander of the 215 Maiwand Afghan Army Corps, told residents to get out as soon as they could.

"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," he added.

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

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

"Deepening concern for Afghan civilians ... as fighting worsens," the United Nations Assistance Mission for Afghanistan (UNAMA) tweeted.

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

"Fighting was intense this morning," said Sefatullah, director of Sukon radio in the city.

Fighting has displaced around 80,000 children since the start of June

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

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 Herat, another city under siege, hundreds of residents chanted Allahu akbar from their rooftops after government forces repulsed the latest Taliban assault.

Washington and London have 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 indulged in revenge killings there, leaving at least 40 people dead.

"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.

Top US diplomat Antony Blinken also attacked the militant leaders.

An Afghanistan without a democratic, inclusive government would be a "pariah state," he said, adding that the international recognition the group wants will not be possible if it "seeks to take the country by force and commits the kind of atrocities that have been reported".

Meanwhile, fighting across the country has displaced around 80,000 children from the start of June, humanitarian organisation Save the Children said, adding that many schools and health facilities had also been damaged.