Health Minister Hamad Hassan, speaking to Al Manar TV, confirmed that the death toll from the blast on Tuesday has risen from 100 to at least 135, with dozens more still missing.
It comes as Foreign Secretary Dominic said the UK's aid to Lebanon was "ready to go".
Mr Raab said the UK is poised to deliver medical experts, humanitarian aid of £5 million, and search-and-rescue experts to Lebanon.
Government schools minister Nick Gibb said all embassy staff based in Beirut are accounted for, but some have suffered “non-life-threatening injuries”.
Mr Gibb said: “All embassy staff are accounted for. A small number have sustained non-life-threatening injuries and, where necessary, are receiving medical attention.
“It is a fast-moving situation and we are monitoring the situation closely.
"We stand ready to offer consular support to British nationals affected. People should check FCO travel advice, which is updated regularly and includes contact details for those requiring consular assistance.”
Sky News has reported that the UK is preparing to send assistance including medics and search and rescue teams.
"We don't know what the assistance is going to be but we understand that it will include search and rescue teams, as well as teams of medics," Sky News' Foreign Affairs editor Deborah Haynes said.
However, when approached by the Standard the Foreign Office refused to comment on whether the report was true.
It comes as the Queen sent a message to the President of the Republic of Lebanon to say she and the Duke of Edinburgh were “deeply saddened” by the news.
The Queen's message said: "Prince Philip and I were deeply saddened by news of the explosion at the Port in Beirut yesterday.
"Our thoughts and prayers are with the families and friends of those who have been injured or lost their lives, and all those whose homes and livelihoods have been affected."
Dominic Raab said the details of Britons caught up in the Beirut blast were still being established.
“We are not sure on the precise figures in relation to UK nationals there, we will obviously want to bottom out that in the days ahead,” the Foreign Secretary said.
“Obviously we have a consular team there which are monitoring that very carefully.”
Speaking outside the Foreign Office, Mr Raab said: “We are ready and now poised to deliver medical experts, humanitarian aid of £5 million, search-and-rescue experts.
“We have also got a Royal Navy survey ship in the area which can be deployed to help assess the damage to the port.
“All of that is ready to go, obviously we will make sure we have got exactly what is tailored towards the Lebanese needs.
Mr Raab said a “full, thorough and rigorous” investigation had been promised by the Lebanese prime minister.
The Foreign Secretary spoke to Hassan Diab on Wednesday to discuss the blast in Beirut’s port.
“He has been very clear with me that there will be a full, thorough and rigorous investigation to get to the truth – I think the people of the Lebanon deserve no less – and that there will be full accountability, subject of course to the results and the outcome of that investigation.”