Hezbollah chief Hassan Nasrallah said investigators were passing information to Israel, in the latest escalation in a war of words over the inquiry which threatens to plunge the country into more turmoil.

Hezbollah stepped up its campaign against the tribunal after Lebanese officials and diplomats said the court's prosecutor may indict members of the group, possibly early next year.

"Any call to boycott the tribunal to prevent it from fulfilling its mandate is a deliberate attempt to obstruct justice," a statement from the office of the prosecutor said.

It said it expected cooperation to continue in accordance with the tribunal's statute and Lebanese laws.

Nasrallah spoke out after two international investigators on Wednesday were forced by a crowd of women to leave a doctor's clinic in southern Beirut, a bastion of Hezbollah, where they had made an appointment to review files.

Tribunal President Judge Antonio Cassese sent letters to UN Secretary-General Ban Ki-moon and Prime Minister Saad Hariri, Rafik's son, conveying the court's "great concern" about Wednesday's incident.

"It is our firm intention not to bow to any act of interference and intimidation in carrying out our mandate in an independent and fair manner," Cassese was quoted as saying in a statement released by the Hague-based tribunal.

Hezbollah, part of a fragile coalition government, has been pressuring Premier Hariri into disavowing the court, which it considers a tool of US and Israeli policy.

The Lebanese government has yet to comment on the issue.

"The situation is definitely at a standoff," said Oussama Safa, director of the Lebanese Center for Policy Studies. "The escalation from Hezbollah has taken new heights, but I think none of this escalation will spiral out of control, nor will the tribunal back down," he told Reuters.

Nasrallah said it was scandalous that investigators had sought the medical files of women at the clinic, and warned that any further cooperation with the tribunal would be considered "an aggression against the resistance (Hezbollah)".

The office of the prosecutor said the visit had been approved by Lebanese authorities and the process leading to the visit "was handled professionally and in accordance with legal safeguards". It also said the investigators were not seeking medical information from the doctor.