BEIRUT: France lambasted the Lebanese authorities Thursday for failing to uncover the causes of a massive explosion that killed more than 200 people and ravaged swathes of Beirut six months ago.

As families of the victims marked half a year since the deadly August 4 port blast, French Ambassador to Lebanon Anne Grillo called for speedy action.

"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.

One of the largest non-nuclear blasts in history was caused by a vast stock of highly explosive ammonium nitrate that had sat for years in a port warehouse, a stone's throw from residential districts.

More than 20 suspects have been detained as part of Lebanon's investigation into the tragedy, but the probe appears nows to be completely stalled.

The kind of gamesmanship that has come to characterise Lebanon's ruling class appears to have blocked the special prosecutor's work.

Former colonial power France led the international response to the tragedy, delivering aid in exchange for promises of political reform and accountability.

French President Emmanuel Macron travelled to Lebanon twice, including two days after the explosion, and has vowed to visit again soon.

Yet his demands for an overhaul of Lebanon's perennially corrupt politics have fallen on deaf ears.

Nearly six months after the government resigned over the port blast, a new cabinet has yet to be formed and Lebanon's economy is left to continue its free fall.

Grillo said that six months on from the explosion, "it is unacceptable that Lebanon should still be without a government to tackle the social and sanitary crises, and begin the structural reforms the country needs to recover and stabilise."