There were no casualties when small bombs damaged the tracks near Karachi, Hyderabad, Nawabshah and Mehrabpur districts, in what appeared to be coordinated attacks, police and railway officials said.

"Two low-intensity bombs planted on both the up and down railway tracks went off outside Karachi early morning, causing damage to the tracks," senior police officer Chaudhry Asad told AFP.

Train services were suspended and repair work had begun, he said.

About 30 minutes later two more blasts damaged railway tracks near Hyderabad city, Pakistan Railways official Aftab Memon said.

Two additional blasts on tracks were reported from Nawabshah district and another two in the Sindh area of Mehrabpur, he said.

"All these blasts were similar in nature, causing damage to the track but no casualties," he said.

Home ministry official Sharfuddin Memon blamed militants for the attacks.

"These attacks are linked to the situation in northwest and the aim was to target communications networks and create panic among the people," he said.

However, attacks by the extremists on infrastructure such as railways are relatively rare.