Sydney beaches have reopened after tar balls washed ashore last week, but the source of the balls remains unknown, the Guardian reports.

The chief executive of NSW Maritime said the pellets appeared to be made of fatty acids, chemicals found in cleaning products and cosmetics, and fuel oil. The mystery has prompted the closure of major beaches in Sydney’s eastern suburbs.

Similar balls appeared on Australian beaches three months ago, but they were black, now they are white and grey. At the time, it was speculated that the balls were formed in sewage, and residents were concerned about their appearance.

Coogee, Maroubra and Clovelly beaches, which remained closed to swimmers, will reopen next Saturday, while Bondi and several other beaches reopened four days ago. However, the cause of the black balls that flooded Coogee Beach earlier this week and prompted a major clean-up operation, is still unknown.

Mark Hutchings, chief executive of NSW Maritime, said that, according to health advice, the balls were not highly toxic to humans.

“We can now confirm that the balls are made up of fatty acids, chemicals similar to those found in cleaning products and cosmetics, mixed with fuel oil,” the statement said.

“They are not harmful when on the ground, but should not be touched or picked up,” the statement added.

An oil spill is the most likely explanation for the formation of the pellets.

Beachgoers are advised to notify a lifeguard if they spot tar balls, and to wash their hands with soap and water or baby oil if they touch them. About 2,000 balls have been collected since the state of emergency began last Tuesday.

“We have not found any further traces of this substance so far, which suggests this is an isolated incident,” Hutchings said. The NSW Environment Agency is still examining the pellets and conducting analyses.

“Until we have completed all the tests, we cannot say for sure where the balls came from,” said Stephen Beaman, the agency’s chief executive.

“It is still a mystery and it may take a few more days to determine the source,” he concluded.

Four days ago, the NSW government promised to take strict action against those responsible for the appearance of the balls.

“We are conducting an investigation to determine the source of the spill and who is responsible,” Environment Minister Penny Sharp said. Tar balls form when oil comes into contact with other waste and water, usually as a result of an oil spill or leak.