Ahmad Al-Shibani told Aljazeera media reports al-Sadr had refused to meet the Iraqi National Conference delegation were false and fabricated.
"On the contrary, Muqtada al-Sadr has welcomed this delegation and has also welcomed any mediation efforts on the part of Pope John Paul II of the Vatican," al-Shibani told Aljazeera.
"However, we are under constant attack by US-led occupation forces and the security atmosphere did not facilitate a meeting with Muqtada al-Sadr".
The Shia leader, his aides and 2000 civilian "volunteers" forming a human shield are in the Imam Ali shrine in Najaf, hemmed in by US-led forces.
The mission is intended to disarm
and transform the al-Mahdi Army
There were unconfirmed attacks the talks with the Baghdad delegation would be rescheduled.
Najaf governor Adnan al-Zirqi said he doubted the reasons given for the delay in the talks, but could not provide any evidence to support his claim.
A scaled-down eight-member delegation of Iraqi political and religious leaders had arrived in Najaf on Tuesday evening to try to persuade Shia leader Muqtada al-Sadr to leave the shrine.
The delegation from the Iraq National Conference (INC), led by US ally and Shia leader Shaikh Husayn al-Sadr, was flown to Najaf by US Blackhawk helicopters on Tuesday, after being delayed twice because of security fears.
The team was originally scheduled to leave on Monday.
The delegation hopes to meet the Shia leader to relay a call from the 1300-delegate INC to disarm his al-Mahdi Army and leave the shrine to end the fighting.
"There are inviolable conditions in civilised countries, particularly that there is no place for armed militias"
Shaikh Husayn al-Sadr,
INC delegation leader
According to one report, the original strength of the delegation numbered 60.
The cut back in the delegation comes amid reports of fighting on Tuesday in Najaf between the al-Mahdi Army and a combination of US occupation and Iraqi security forces.
The delegation entered the huge shrine compound at about 7am (1500 GMT) at the heart of the city's historic centre, where continuous, heavy gunfire raged throughout the day, punctuated by mortar blasts.
"This is a peace initiative... We're not going to negotiate or lay conditions on Sayyid Muqtada," Shaikh Husayn said.
"The delegation is just going to deliver the message issued by the Iraqi National Conference, which includes transforming the al-Mahdi Army into a political organisation, vacating the holy shrine area and an invitation to Sayyid Muqtada and his followers to take part in the political process," he added.
No to militias
On Monday, participants said they were optimistic they would be able to convince al-Sadr to quit the shrine.
Al-Sadr loyalists continued to
fight US-led troops on Tuesday
Delegates have approved a proposal put forward by Shaikh Husayn, who told the conference: "There are inviolable conditions in civilised countries, particularly that there is no place for armed militias."
Talking to Aljazeera earlier, INC member Abbas al-Bayati said one of the key points to be discussed with al-Sadr would be to disarm the al-Mahdi Army and transform it into a political organisation.
Al-Sadr aide al-Shaibani said the Shia leader had agreed to receive two delegations - one headed by the UN secretary-general's representative in Iraq and the other from the INC.
Al-Sadr had expressed readiness to negotiate turning his group into a political organisation, but rejected the call for it to lay down arms, which he described as being for personal use, al-Shaibani said.