The Taliban on Friday captured their first provincial capital since launching an offensive to coincide with the departure of foreign troops, a major blow to an Afghan government desperately trying to push back the insurgents.

"Zaranj, provincial capital of Nimroz, has fallen to the Taliban," Roh Gul Khairzad, the deputy governor of Nimroz, told AFP.

She said the city -- in southwest Afghanistan near the Iranian border -- had been taken "without a fight", and social media showed clips of insurgents roaming the streets, being cheered by residents.

The veracity of the videos could not immediately be confirmed.

The fall of Zaranj comes the same day the Taliban claimed responsibility for killing the head of the Afghan government's media information department.

The insurgents warned this week they would target senior administration figures in retaliation for increased air strikes.

The assassination on Friday of Dawa Khan Menapal, one of the government's leading voices, followed another bloody day of fighting as the war increasingly spills into Kabul.

The news from Nimroz also comes as the UN Security Council met in New York to discuss the conflict.

Deborah Lyons, head of the world body's Afghan aid operation, painted a grim picture of the country's deteriorating situation.

"The Security Council must issue an unambiguous statement that attacks against cities must stop now," Lyons said via video-link from Kabul.

Cowardly act

In the Afghan capital, officials expressed shock at the fatal shooting of Menapal.

"Unfortunately, the savage terrorists have committed a cowardly act once again and martyred a patriotic Afghan," said interior ministry spokesman Mirwais Stanikzai.

Menapal was popular in Kabul's tight-knit media community, and known for pillorying the Taliban on social media -- even jokingly at times. 

Taliban spokesman Zabihullah Mujahid sent a message to the media saying Menapal "was killed in a special attack carried out by mujahideen".

The murder comes after the militants warned Wednesday of more attacks targeting Afghan government leaders. 

The day before, defence minister Bismillah Mohammadi escaped an assassination attempt in a bomb-and-gun attack.

The Afghan and US militaries have stepped up air strikes in their fight against the insurgents in a string of cities, and the Taliban said Tuesday's Kabul attack was their response.

Fighting in Afghanistan's long-running conflict has intensified since May, when foreign forces began the final stage of a withdrawal due to be completed later this month.

The Taliban already control large portions of the countryside, and are now challenging government forces in several provincial capitals.

Nothing left

Government forces continue to hit Taliban positions with air strikes and commando raids, and the defence ministry boasted Friday of eliminating more than 400 insurgents in the past 24 hours. 

Both sides frequently exaggerate battlefield casualty figures, making independent verification virtually impossible. 

But even as Afghan officials claimed to be hitting the Taliban hard, security forces have yet to flush out the militants from provincial capitals they have already entered -- with hundreds of thousands of civilians forced to flee in recent weeks.

Social media was also filled with videos of the devastating toll the fighting has taken in the southern city of Lashkar Gah -- capital of Helmand province, with posts showing a major market area in flames. 

Aid group Action Against Hunger said its offices had been hit by an "aerial bomb" in the city earlier this week, according to a statement released by the organisation on Friday.

"The building was marked from the street and roof as a non-governmental (NGO) organisation, and the office location has been communicated often to the parties involved in the conflict," said the group, adding that no staff had been harmed.

In the key western city of Herat, a steady stream of people were leaving their homes in anticipation of a government assault on positions recently won by the Taliban.

"We completely evacuated," resident Ahmad Zia told AFP. "We have nothing left and we do not know where to go."