Washington, Oct 20 : The United States has offered assistance to Pakistan in response to the "horrible tragedy" in Karachi, where suicide bombers targeted former Prime Minister Benazir Bhutto at her reception rally killing at least 139 people, a State Department spokesman has said.

Spokesman Tom Casey assured the Pakistani officials of the US assistance, in the wake of suicide attacks on Bhutto.

The US envoy "also called Bhutto just to convey our sympathy and the sympathy of the US Government for those individuals whose lives were lost and also to express our concern for her well-being as well," Casey added.

Replying to a question, Casey said Pakistani security services were protecting Bhutto and there was no US involvement in it.

Commenting on the threat posed by extremism in Pakistan and elsewhere, Casey said that it is another reason why "we all need to work together to confront it, because certainly I don't think the people of Pakistan wish to see people trying to express political views through bombs and bullets."

"We are working with them, and I know that part of the efforts of many of the political parties in Pakistan is to be able to help themselves have a free and open and transparent electoral system. That's the way you choose leaders, not through political assassination," he added.

Casey also welcomed the Pakistani government's pledge to fully investigate the bombing.

"I do again take as positive the fact that the Pakistan Government has promised a full investigation, that Bhutto, again, has praised the efforts of the security forces who were protecting her."

Casey cited reports that Musharraf in fact called Bhutto about this to express his concerns as well and noted that " Bhutto in fact did commend the work being done by the security personnel that were with her, many of whom, again, were victims of this as well."

The US, he said, hopes to see a thorough investigation into the incident.

"We do not know who is responsible for this terrible attack, but we hope that the individuals who are, are found and brought to justice."

Meanwhile, the White House Friday blamed "radical Islamic extremists" for the attack on Bhutto, The Nation reported.

"We continue to get the reports back. Obviously, it was a horrible tragedy, with over, I think, 136 people being murdered, innocent people that were there just to rally around someone that they support," Press Secretary Dana Perino said.

She said President George W. Bush is "concerned that overall -- when you look at a moderate Islamic state like Pakistan, that any time they try to make moves towards stabilising their democracy, that radical Islamic extremists do something in order to try to stop that progress, and that is a concern to the President not only in Pakistan, but in the entire region."

Continuing, she said, one of the reasons that the US is "actively engaged is to try to make sure that we can stop this trend of radical Islamic extremists from murdering innocent people, especially the women and children -- men, women and children who were there." (ANI)