The fuel for the massive explosion that ripped through Beirut on Tuesday was brought to the Lebanese capital by a Moldovan-flagged cargo ship, maritime crime watchdog Stable Seas said.

The 3,226-dwt Rhosus (built 1986) was laden with 2,700 tonnes of ammonium nitrate destined for Mozambique in 2013 when it was diverted to Beirut, detained by port authorities over deficiencies and later abandoned by its owner.

"The consequences of ship and seafarer abandonment are rarely as visible and shocking as they were in Beirut on Tuesday, but the problem is actually quite common," Stable Seas said, noting the Covid-19 pandemic was causing more ships and seafarers to be abandoned.

After the Rhosus' arrest, Stables Seas said, most of its crew were repatriated, but the captain and three seafarers stayed behind and were unable to disembark due to immigration issues.

In the proceeding months, attorneys from local firm Baroudi & Associates were able to convince a judge to allow the seafarers to disembark.

The highly explosive cargo, primarily used in agriculture, was reportedly placed in a nearby warehouse, where it stayed until it was ignited Tuesday.

It is still unclear what caused the blast, which was felt as far away as Cyprus, levelled the port and damaged buildings as far as 5 km away.

On Wednesday, officials said the death toll had risen to 135, with 5,000 injured.

In the blast's aftermath, Lebanese Prime Minister Hassan Diab issued a statement saying those responsible "will pay the price".

"I promise that this catastrophe will not go unpunished and those responsible will be held accountable," he said.

According to shipping databases, the Rhosus' registered owner is Briarwood Corp, which shares an address in Bourgas, Bulgaria, with the ship's manager, Interfleet Shipmanagement.

Contact information was not immediately available for either company.

It is unclear what happened to the Rhosus after its abandonment.

Shipping databases say the ship is still in service, but its AIS' last broadcast was on 7 August 2014 when it was being held in Lebanon.