In a televised address to the nation, Diab said: "What happened today will not pass without accountability. Those responsible for this catastrophe will pay the price.”
Diab also appealed for international assistance to help Lebanon, which is suffering its worst economic crisis in decades.
France was among the nations which responded to the call.
French foreign minister, Jean-Yves Le Drian, said in a tweet: "France stands and will always stand by the side of Lebanon and the Lebanese."
La France se tient et se tiendra toujours aux côtés du Liban et des Libanais. Elle est disposée à apporter son assistance en fonction des besoins qu’exprimeront les autorités libanaises.
— Jean-Yves Le Drian (@JY_LeDrian) August 4, 2020
Israel’s defence minister, Benny Gantz, and foreign minister, Gabi Ashkenazi, issued a joint statement offering medical and humanitarian aid via international intermediaries.
Gulf nations also responded to the appeal. Qatar and Kuwait promised to send field hospitals to support the medical response.
"Our thoughts and prayers are with the great and resilient people of Lebanon," Iran’s foreign minister Mohammad Javad Zarif tweeted. "Stay strong, Lebanon."
Blasts
The initial explosion happened just after 6pm local time. Video footage of the second blast showed an enormous orange fireball that consumed nearby buildings and sent a shockwave through the city.
General security chief Abbas Ibrahim said the blasts may have been caused by explosive materials confiscated years ago and stored at the city's port.
"We heard an explosion, then we saw the mushroom," said one resident who witnessed the second explosion from her balcony in the city's Mansourieh district.
"The force of the blast threw us backwards into the apartment," she added.
The Lebanese media carried images of people trapped under rubble.
Injured survivors roamed the streets while outside the Clemenceau Medical Centre, dozens of wounded waited for treatment.
The port zone was cordoned off by the security forces, allowing access only to ambulances and fire trucks.
