The United States and France on Thursday emphasised the "urgent need" for Lebanon's leaders to form a "credible and effective government", six months after a blast devastated Beirut's port.

In a joint statement, US Secretary of State Antony Blinken and French Foreign Minister Jean-Yves Le Drian said the formation of a government and enactment of reforms were "crucial" for the ongoing support of international donors.

"The six-month anniversary of this tragic event underscores the urgent and vital need for Lebanese stakeholders to finally act upon the commitments they have made to form a credible and effective government, and to pave the way for the implementation of necessary reforms, in accordance with the aspirations of the Lebanese people," the two top diplomats said.

They said they also expected "rapid results" from the investigation into the causes of the blast, which killed more than 200 people and gutted a large part of southern Beirut.

France's ambassador to Lebanon, Anne Grillo, earlier expressed frustration at the delay in shedding light on the disaster.

"Six months after the explosion, it is unacceptable that the Lebanese people should still be waiting for answers from its leaders," she said in a statement.

Lebanon's cabinet stepped down after the August 4 explosion, but efforts to form a new one have since hit a wall.

French President Emmanuel Macron, who has visited Lebanon twice since the blast, has conditioned the triggering of a massive aid plan on the formation of a reform-minded government.

The French government is hoping for backing on the issue from new US President Joe Biden's administration.