Montas said Ban Ki-moon, the UN secretary general, would send a delegation to the Netherlands in the next few weeks to discuss practical arrangements for the tribunal, whose establishment has been authorised by the UN Security Council.
The letter of Jan Peter Balkenende,the Dutch prime minister, arrived on Wednesday, she said.
Al-Hariri and 22 others died in February 2005 in a Beirut car bomb blast that interim UN findings have linked to Syrian and Lebanese security officials.
Syria has denied involvement, but the outcry forced it to withdraw its troops from Lebanon.
The United Nations and the Lebanese government agreed last year that a special tribunal based outside Lebanon would try those suspected of killing al-Hariri and others implicated in a spate of political assassinations.
Maxime Verhagen, the Netherlands foreign minister, told Dutch radio station Evangelische Omroep on Thursday that the agreement still needed to work out who would bear the costs for the tribunal.
The United Nations announced in July it had asked the Netherlands to host the tribunal, a step it rarely takes unless agreement had been assured.
No suspects named
At the request of Fuad Siniora, the Lebanese prime minister, the UN Security Council voted to set up the special tribunal on June 10, despite opposition from anti-government parliamentarians in the divided country.
UN officials have said they expect it to take up to a year to get the court functioning after a UN-established commission completes its investigation.
UN investigators probing the killing have identified a number of people who may have been involved or known about it, their chief reported this month.
Prosecutor Serge Brammertz of Belgium did not name any suspects in a report to the Security Council last month, which also expressed concern that deteriorating security in Lebanon could hamper the inquiry.
His predecessor, Detlev Mehlis of Germany, had suggested Syrian and Lebanese intelligence services were involved.
Brammertz also is investigating 17 other political murders or attempted murders in Lebanon.