Travellers, many of whom are already confused about carry-on restrictions on liquids and gels, will now have another thing to worry about - powders.

New rules introduced by the United States, Australia and New Zealand - with effect from June 30 - will affect many travellers, including those on more than 30 flights a day to and from Singapore.

The restrictions, that appear to have been prompted by a foiled plot to bomb an Etihad Airways plane flying from Sydney to Abu Dhabi last year, apply to all flights to the United States, and from Australia and New Zealand to Singapore.

Travellers on these flights are not allowed to carry more than 350ml of "inorganic powders" like salt, talcum powder or sand in their hand luggage.

While there is no limit on carrying organic powders such as food in powdered form and powdered baby formula, as well as human remains, these items must be presented to officers during security screenings.

In a notice on its website, Singapore Airlines (SIA) has advised customers to put all the items inside a properly sealed bag - the same bags that are used to carry liquids, gels and aerosols.

For affected flights from Singapore - SIA's SQ32, United Airlines' UA2 and UA38, and Delta Air Lines' DL166 - passengers may be asked to go through an additional security check for powdered items after the standard security screening procedures at the boarding gate, Changi Airport Group spokesman Ivan Tan said.

He told The Straits Times that resources have been deployed to conduct the additional security checks since June 30.

In Australia, the restrictions also apply to domestic flights departing from an international Australian terminal. This means that a traveller from Singapore flying to Sydney, for example, and from there to another Australian destination, must make sure the restricted items are correctly packed.

Both SIA and Australia's Qantas have advised affected travellers to proceed to their boarding gates early to allow sufficient time for the new restrictions.

Supporting the new restrictions, the International Air Transport Association (Iata) said that the security of passengers, crew, and airport workers is paramount for everyone involved in aviation.

"The International Civil Aviation Organisation (ICAO) and member states have identified an increased level of threat from chemical, biological and radiological elements in powders carried by passengers," said a spokesman for the body that represents global carriers.

"The industry has been widely consulted and has full confidence in the measures that governments are taking... which we believe will have a limited impact in terms of flight disruptions," he added.

Asked if Singapore was planning to introduce similar restrictions, a spokesman for the Ministry of Transport said it works closely with the Singapore Police Force and relevant government agencies and industry stakeholders to regularly review the aviation security requirements applied for flights departing from and arriving at Changi Airport.