United Nations: The United States on Wednesday pledged $10 million for a UN tribunal that is investigating the killing of former Lebanese Prime Minister Rafik Hariri in 2005 and whose funding has provoked a political standoff in Lebanon.

Lebanon's Hezbollah group is trying to block the court's financing and has called on Lebanese not to cooperate with it. Hezbollah is in the country's fragile national unity government and is expected to be the focus of indictments from the tribunal in the coming months.

The $10 million donation brings total US funding for the court to $30 million, according to Susan Rice, the US ambassador to the United Nations.

"We are confident that the work of the tribunal can continue to help deter further violence and put an end to a tragic era of impunity for political assassinations in Lebanon," Rice said in a statement.

"Until Lebanon is able to achieve this, it will be very difficult to secure the peace and stability that all Lebanese citizens deserve," Rice added.

Hezbollah, which is backed by Iran and Syria, 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.

The killing of Hariri and 22 others in a suicide truck bombing sparked an international outcry against Syria, forcing it to end a three-decade military presence in Lebanon. Syria and Hezbollah deny any involvement in the attack.

Lebanon is responsible for 49 per cent of the tribunal's budget, which was envisaged at around $40 million per year when it was set up in 2007.

The court has yet to secure full funding next year and the entire Lebanese national budget has been held up in parliament due to political wrangling over the tribunal.

The 2007 resolution that established the court says that if the Lebanese government's contributions are insufficient, UN Secretary-General Ban Ki-moon can accept "voluntary contributions from states to cover any shortfall."

The tribunal has also condemned what it called an "attack on its staff" last week and said that it would not be deterred from its investigation.

Hezbollah has been trying to pressure Prime Minister Sa’ad Hariri, the slain Hariri's son, into repudiating the tribunal, which the group considers a tool of US and Israeli policy.

Hariri reiterated his support for the UN investigation into his father's assassination in an interview with BBC radio that he gave this week during a visit to London.