Hong Kong authorities have made their third big seizure of illegal ivory in three months, confiscating more than a tonne of elephant tusks worth $1.4m, customs officials said on Friday.

Customs officers seized 779 pieces of ivory weighing 1,323kg (2,916lb) in a shipping container that arrived at Hong Kong's port from Kenya after passing through Malaysia.

The officers discovered the ivory after x-raying the container, which was declared to be carrying architectural stones. Forty sacks holding the ivory were found inside five wooden crates, hidden under rocks.

The investigation is still continuing. No one has been arrested.

The discovery on 20 October of nearly four tonnes in two shipments, worth $3.4m, was the city's biggest seizure in a single operation. A shipment found in November was about the same size as the one found on Thursday.

Hong Kong authorities also confiscated a shipment of ivory worth $2.2m in 2011.

Wildlife activists say strong demand and high prices for ivory, which can fetch up to $1,000 a pound, have driven elephant poaching to record levels. Most illegal ivory is believed to be obtained through the wildlife trade in Africa and smuggled to China and Thailand for use as ornamental items.