A probe into a major scandal threatening Pakistan's president on Tuesday appeared at risk of collapsing as investigators ruled out travelling abroad to hear the testimony of the star witness.

American businessman Mansoor Ijaz, who implicated President Asif Ali Zardari in a May memo seeking US help to rein in Pakistan's powerful military, has refused to travel to Pakistan, citing fears for his safety.

His testimony is considered key to any case against the president, who faces frenzied speculation that he could be forced out of office over the scandal.

Giving Ijaz another opportunity to appear, a three-member judicial commission adjourned until February 9 and said they would ask the Supreme Court to extend their mandate, which was due to end on Saturday.

The Supreme Court ordered an investigation into the scandal following advice from the head of Pakistan's intelligence agency.

Zardari's ambassador to Washington, Husain Haqqani, was forced to resign over the scandal but he flatly denies writing the memo. The courts have since put restrictions on his ability to leave Pakistan.

"The commission cannot go abroad because the Supreme Court has restrained Husain Haqqani, so in the same way, Mansoor Ijaz's statement should be recorded in Pakistan," said Justice Qazi Faez Isa.

The judges earlier summoned Interior Minister Rehman Malik to explain how the government could protect Ijaz.

"He will be given box security. I assure you that his name will not be put on exit control list," Malik told the commission.

"It is better for the image of the country that anyone coming here to give evidence, goes back safely," Justice Qazi Faez Isa told Malik.

Ijaz's lawyer, Akram Sheikh, earlier told the commission that his client feared being barred from leaving Pakistan. Instead, he has offered to record his testimony in London or Zurich -- an option thrown out by Justice Isa.

Isa said the commission needs to cross-examine Ijaz and also examine all the electronic devices.

Sheikh said his client feared for his life if he visited the country and does not trust the Interior Minister. "He thinks Rehman Malik will kill him".

Ijaz's lawyer said Malik was the head of Benazir Bhutto's security team when she was killed in a gun and suicide attack in 2007, a charge Malik denied.

"This is wrong. The then government was responsible for her security and I was not in the government at that time," Malik said.

The memo was delivered on May 10 to Admiral Mike Mullen, chairman of the US Joint Chiefs of Staff at the time, and allegedly aimed to forestall a feared military coup after American troops killed Osama bin Laden in Pakistan.

Writing in the Financial Times on October 10, Ijaz alleged that a senior, unnamed Pakistani diplomat telephoned him asking for help because Zardari needed to communicate an urgent message to the Americans.

"Mansoor Ijaz is the key person and for me. This case is zero without his statement," Pakistani analyst A.H. Nayyar told AFP.

"If he is not willing to come (to Pakistan) all this means nothing."