KARACHI - Two bombs exploded near buses carrying navy officials in Pakistan's southern city of Karachi on Tuesday, killing four people and wounding 56, officials said, the first major attack on the military in seven years in the city.

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Police said the bombs targeting the buses in two different parts of the city exploded simultaneously using remote controlled devices.  

A junior naval officer and a civilian female doctor were among those killed in the twin blasts, Pakistan Navy spokesman Commander Salman Ali said.

Nobody has claimed responsibility for the attacks yet, but analysts said that the attacks may be part of a wider militant campaign to hit security forces across the country.

"It appears to be part of the same militant campaign but I don't see any logic in targeting the navy because unlike army and air force they are not  involved in any operations against the militants," said Tasneem Noorani, a security analyst and former interior secretary.

"They may have targeted navy out of desperation because the other forces (air force and army) may have become very careful and are difficult to attack."

The attack on the military in Karachi was the first since 2004 when gunmen ambushed a convoy escorting the Karachi army corps commander. The general narrowly escaped that attack.

Karachi is Pakistan's biggest city and commercial hub. It hosts the country's central bank, the main stock exchange as well as the two main ports. Most foreign companies working in Pakistan have offices in Karachi, which is also the main base for the navy.

"Explosive devices were planted on the road and they exploded as the buses were passing. Both buses were carrying navy officials," senior police officer Iqbal Mahmood said.  

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.

Separately,  gunmen attacked and set fire to a bus in Pakistan's southwestern Baluchistan province late on Monday, burning 15 people to death, including four children and two women, a senior government official said.

The attack took place in Sibi town, about 160 km east of the provincial capital of Quetta when the bus was parked at a roadside restaurant.

Ethnic Baluch militants have waged a low-level insurgency for decades for more autonomy and greater control of natural resources of their region. They frequently attack government installations and security forces in their violent campaign.