All-rounder Scott Styris pulled out on Thursday

Specific threats were made against the players prompting four to pull out and others to express doubts.

Full details of the threat have not been released but NZC chief executive Martin Snedden said it was "explicit about the Black Caps in Pakistan."

He said an e-mail "certainly threatened consequences if the team were to ignore the warning."

Craig McMillan, Scott Styris, Lou Vincent, Ian Butler have all said they will not travel to Pakistan.

Heath Mills, manager of New Zealand Players Association, said other team-mates were "definitely shaken" by the threat.

Snedden said officials were gathering as much information as they could about the threat.

He insisted it was still "a bit difficult to know" if the tour will go ahead.

But the Pakistan Cricket Board has warned that if the Kiwis do not go ahead with the series, they will cancel a trip to New Zealand in December in retaliation.

"If they are not going to fulfil their obligation, we are not going to tour New Zealand," said chairman Tauqir Zia.

Bomb

The Black Caps cut short their tour of Pakistan following a bomb outside their Karachi hotel last year.

The country became a no-go zone for other cricketing nations until Bangladesh toured in August this year, and South Africa followed last month.

South Africa avoided Karachi and the northwest city Peshawar on the grounds of risk and the team was flanked by presidential-level security squads.

Pakistan Cricket Board (PCB) spokesman Samiul Hassan said: "Security is perfect."