At least 78 people were killed and 4,000 wounded in a massive explosion that shook the Lebanese capital, Beirut, on Tuesday, Lebanon's health minister told Reuters.

It's still unclear what exactly caused the explosion, centered on the city's port area. It was earlier blamed on a fire that broke out.

According to the BBC, officials are blaming highly explosive materials stored in a warehouse for six years.

Lebanon's Prime Minister reportedly said an investigation would focus on an estimated 2,750 tons of the explosive ammonium nitrate, stored at a warehouse for six years.

An investigation is under way to find the exact trigger for the explosion.  Lebanon's Supreme Defense Council said those responsible would face the "maximum punishment" possible.

CNN says the blast felt like an earthquake, while one witness said she had never felt an explosion like it in the city.

President Aoun declared a three-day mourning period, and said the government would release 100 billion lira (equivalent to US$66 million) of emergency funds.