SYDNEY -- Australia's defense force operations to protect its sea trade routes, including through the South China Sea, are becoming more risky as Beijing undertakes the "biggest military build-up in the world today", Australia's defence minister said on Tuesday.

Open sea lanes, including trade routes that go through the South China Sea and East China Sea, are at the core of Australia's national interest, Richard Marles said in an opening speech at a navy conference in Sydney.

"That work is challenging and in truth it is becoming increasingly risky. The biggest military build-up in the world today is China," he told the Indo-Pacific conference.

"That it is happening without strategic reassurance means that for Australia and so many countries a response is demanded."

About 100 protesters, including pro-Palestinian groups, gathered outside the conference centre in Darling Harbour in Sydney. New South Wales state police said 10 people were arrested and pepper spray was used after clashes with officers.

Several Israeli companies are exhibiting at the defense conference.

Marles said Australia was increasing its military spending to build a "more capable, lethal, long-range navy".

This included acquiring frigates from Japan, developing submarine drones with U.S. company Anduril, and expanding its naval shipyards facing the Indian Ocean.

Australia raised concerns with Beijing last month after a Chinese fighter jet dropped flares near an Australian maritime patrol plane carrying out surveillance in the South China Sea, the latest in a series of such incidents that Australia has labelled "unsafe and unprofessional".