(Updates with background)

By Katharine Houreld

KABUL, March 11 (Reuters) - A gunman shot dead a Swedish journalist outside a restaurant in a brazen attack in one Kabul’s most heavily guarded districts on Tuesday, police and embassy sources said, underscoring growing insecurity threatening next month’s elections.

The Swedish Embassy identified the victim as Nils Horner, 51, who worked for Swedish Radio and had dual British-Swedish nationality.

“Nils was one of our absolute best and most experienced correspondents and what has happened to him today is terrible,” said Swedish Radio’s director-general, Cilla Benkö, who described this as one of the worst days in the corporation’s history.

“We are now trying to get as many details as we can.”

Horner had been waiting outside a Lebanese restaurant with his driver and translator when two men in Western clothes approached and one shot him at point-blank range in the back of the head, said Zubir, a guard at the restaurant who uses only one name.

The guard and a nearby shopkeeper said only one shot was fired.

The attack took place barely a minute’s walk from the site of another Lebanese restaurant, where Afghan Taliban fighters killed eight Afghans and 13 foreigners in January.

Zabihullah Mujahid, a spokesman for the Taliban, who are seeking to oust foreign forces and set up an Islamic state, said the group was unaware of the attack but would investigate.

Hashmatullah Stanekzai, chief spokesman for the Kabul police chief, said Horner’s driver and translator were being questioned but there were no suspects in custody.

The neighborhood is home to several embassies, supermarkets and cafes frequented by foreigners. Police vehicles are permanently stationed at a roundabout a block away and the mansions that line the road have guards at each gate.

A daytime attack on a civilian walking in that part of the capital is highly unusual.

The attack comes as Afghanistan prepares for the withdrawal of NATO forces and landmark presidential elections scheduled for April 5. The Taliban have threatened to attack anyone who takes part.

Afghan troops with support from NATO are helping secure the elections.

A small contingent of Americans may remain behind if the next government signs a deal to allow them to stay, something President Hamid Karzai has so far refused to do. (Additional reporting by Jessica Donati, Hamid Shalizi and Mirwais Harooni; Editing by Nick Macfie)