Pakistani security agents have arrested a youth who said he had been trained as a suicide bomber to kill opposition leader Benazir Bhutto had other militants failed to kill her, in the first arrests in the case, a security official said.

The 15-year-old and his militant "handler" were arrested in the northwestern town of Dera Ismail Khan on Thursday night.

The teenager, identified as Aitezaz Shah, told interrogators he would have been "next in line" to kill Bhutto if another team had failed on December 27, the security official said on Saturday.

Former prime minister Bhutto was killed in a gun and bomb attack as she left an election rally in the city of Rawalpindi on December 27.

The youth told interrogators he prepared for his mission in a stronghold of al Qaeda-linked militant leader Baitullah Mehsud in the South Waziristan region on the Afghan border, said the official who declined to be identified.

The official said the "handler", identified as Sher Zaman, was the more important catch. "He is the real catch who can give us more clues," the official said.

The government said Mehsud was behind Bhutto's killing. CIA Director Michael Hayden, in an interview with the Washington Post published on Friday, blamed Mehsud for Bhutto's killing.