KANDAHAR, Afghanistan — Residents were urged on Tuesday to evacuate a besieged Afghan city as the army prepared a major offensive against Taliban insurgents after three days of heavy fighting.

A massive blast meanwhile rocked Kabul on Tuesday evening, sending a thick plume of smoke into the sky, AFP correspondents reported.

It appeared to take place in the centre of the city but no details were immediately available.

The Taliban have seized control of much of rural Afghanistan since foreign forces began the last stage of their withdrawal in May, but 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 were 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.

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.

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

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

'Taliban are everywhere' 

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

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.

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, hundreds of residents chanted "Allah-u Akbar” (God is greatest) from their rooftops on Monday night after government forces countered the latest Taliban assault.

Officials said government forces had managed to push back the insurgents from several parts of Herat — including near the airport, which is vital for resupplies.

But on Tuesday afternoon four rockets struck the airport leading to cancelling of two flights although the facility was not damaged, airport chief Shaheer Salehi told AFP.

‘War crimes’ 

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 indulged in 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.

US Secretary of State Antony Blinken said the international recognition the Taliban want will not be possible if they seek "to take the country by force and commits the kind of atrocities that have been reported”.

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