Further slides in the wake of the deadly collapse slowed rescue efforts at the quarry in Ibri, Al-Dhahirah governorate, the Gulf state's Civil Defence and Ambulance Authority said.

Footage posted by the CDAA showed a towering cliff face crumbling onto the quarry below, creating a huge cloud of dust as workers shouted below.

"The rockslides are still going on and hampering the search for missing people," it tweeted.

"So far, six dead people have been recovered and four others have been treated in moderate to critical condition," it added, without giving the victims' nationalities or the cause.

State television later showed trucks and tractors at the rescue site, as Oman's labour union called on Twitter for an inquiry to ensure employers were respecting security regulations.

Ibri, in the Hajjar mountain chain about 200 kilometres (125 miles) from the capital Muscat, is a centre for quarries, mainly marble, which often employ immigrants from South Asia.