International aid flights are arriving in Lebanon, as the government struggles to deal with the aftermath of the massive blast that killed at least 135 and sent thousands more to hospital.

Investigators are looking into why nothing was done about 2,750 tonnes of highly explosive ammonium nitrate stored at the port for six years.

Search and rescue teams, medical personnel, humanitarian support packages and equipment designed to find trapped victims are being sent from countries like Australia and Indonesia as well as Europe.

Russia's Ministry for Emergencies says that its first plane carrying relief teams, doctors and medical equipment has landed in Beirut.

The ministry said Wednesday the aircraft has delivered a mobile hospital along with 50 emergency workers and medical personnel.

Another three Russian flights are scheduled to arrive within the next 24 hours.

They will carry equipment for a coronavirus testing lab and protective gear, among other relief supplies.

The airlift follows a request for help from the Lebanese authorities faced with the aftermath of the massive explosion that devastated Beirut.