The building, at a university in Qassim northwest of the capital Riyadh, collapsed on Monday.
The bodies of six Pakistanis had been pulled from the rubble and rescuers were trying to free the bodies of two more, the diplomatic source told AFP, speaking on condition of anonymity.
The diplomat said three other people wounded in the collapse had also died, including two more Pakistanis and an Indian.
A spokesman for the Saudi Red Crescent Authority gave a different toll, saying six people had died, after providing an initial count of two dead late on Monday.
The spokesman did not give the nationalities of the victims.
Sniffer dogs were assisting in the search for up to six other workers believed still trapped under the concrete.
The diplomatic source said he understood that steel mesh had been laid and foundation columns were in place.
“When they started pouring concrete, that was when the load was too much” for the columns, he said.
Pictures from the scene showed a sprawling but low-level construction site, where many steel support rods had collapsed along with other parts of the structure inside a wide circular area.
Millions of foreign workers from impoverished countries in Asia are employed at a variety of jobs in Saudi Arabia, including construction work.
The diplomatic source said Egyptians and Bangladeshis also worked on the site.
Pakistan confirms death of six nationals
According to country’s Foreign Ministry, six Pakistanis have been confirmed dead in the unfortunate incident of collapse of an under-construction building at Al-Qassim University in Buraidah. Seven Pakistanis sustained injuries. One of them has been released from the hospital, while the other six are in stable condition.
According to the Chief Executive of the Manazel Construction Company, Mr Ahmad Othaim, two Pakistani workers are still missing.
Embassy officials along with a legal consultant are present at Al-Qassim to coordinate and supervise arrangements for the treatment of our injured citizens and for dispatch of dead bodies to Pakistan.
“The Ministry of Foreign Affairs and our Embassy in Riyadh will continue to monitor the situation. The families of Pakistanis employed by Manazel Construction Company can contact the Embassy of Pakistan in Riyadh at 00966-571870352 & 00966-571862623.”