The bus was ferrying naval officials to work when the explosion occurred [AFP]

A blast has hit a bus carrying Pakistani navy officials in the southern city of Karachi, killing five people, officials have said.

"Now a total of four of our employees - all sailors - have been martyred in the attack on our bus while seven others are injured," navy spokesman Commander Salman Ali told AFP.

A hospital official said a passerby was also killed in the incident, which took place in the city's busy Faisal avenue.

About three kilogrammes of explosives were packed into the bomb which was detonated remotely, senior police official Iftikhar Tarar said.

Karachi is Pakistan's commercial hub and also home to the navy's main base.

Thursday's blast occurred two days after two other navy buses were attacked in the city. Four people were killed in those blasts and 56 others wounded.

Both the Baluchistan Liberation Army and the Pakistani Taliban claimed responsibility for Tuesday's attacks, which police said were carried out with remotely controlled bombs roughly 15 minutes apart in different areas of the city.

The Taliban vowed to continue attacks on the country's military until it stops targeting the group in the country's northwest.

Fighters have not targeted the navy in the past and security experts say the attacks on the navy, seen as a soft target with less effective protection, could be part of a new strategy to widen their violent campaign.

"It's not the navy that is being targeted, it is the armed forces," Moinuddin Haider, a former interior minister and retired army general, said.

"And the most dangerous thing is that IEDs [improvised explosive devices] and remote bombs are now being used. This is a new thing which here first started in Iraq and has caused a lot of damage in Afghanistan."

The attacks this week on the military in Karachi were the first since 2004 when assailants ambushed a convoy escorting the Karachi army corps commander. The general escaped that attack.

In 2002, 11 French engineers and technicians working on the construction of submarines for the Pakistani navy were killed, along with three Pakistanis in a suicide car bombing outside a hotel in Karachi.

Source: Al Jazeera and agencies