Lebanon's investigating judge has charged caretaker Prime Minister Hassan Diab and three former ministers with negligence over the Beirut port blast that killed 200 people and ruined a broad area of the capital in August.

The others are ex-finance minister Ali Hassan Khalil, as well as former public works ministers Ghazi Zeaiter and Youssef Finianos, state news agency NNA said.

Mr Zeaiter headed the public works and transport ministry in 2014, shortly after the arrival of the Rhosus ship carrying a huge stockpile of ammonium nitrate which detonated at the port on 4 August.

Officials have said the cargo of ammonium nitrate, a highly explosive chemical, was stored unsafely for years at the port, which lies in the heart of Beirut.

Mr Diab said in a statement that his conscience was clear over the blast.

He said he was confident that his hands were clean and that he had dealt transparently with the file of the explosion, adding that he was surprised to be targeted by the investigating judge.

Four months after one of the largest non-nuclear blasts on record, which injured thousands of people, victims are still waiting for the results of an investigation their leaders had promised would come within days of the disaster.

The judge leading the official inquiry, Fadi Sawan, sent a letter last month asking parliament to investigate 10 former ministers.