China's national legislature declined to announce the country's 2013 defence budget at a press conference Monday, in a departure from normal practice the day before its annual session opens.

Fu Ying, spokeswoman for the National People's Congress (NPC), which begins Tuesday, responded with frustration when she was asked about military spending at a press conference where in recent years the budget has been revealed.

"It seems China has to explain every year to the outside world why we should strengthen national defence and why we should increase the military budget," Fu said.

China's defence budget has been announced at the pre-NPC press conference going back to at least 2006. Last year the then NPC spokesman said 2012 military spending would rise 11.2 percent to 670.27 billion yuan ($107.6 billion now).

The announced budgets are subject to formal approval by the NPC, and may emerge during its sessions on government finances in the coming days.

Experts say that China's actual defence spending is usually substantially higher than the publicly declared figures.

China's military budgets have risen steadily in recent years along with the country's booming economic growth that has seen it catapult past Japan to become the world's second-largest economy.

But the issue has become a sensitive one as the rises have come with China taking what some neighbouring countries see as a more assertive military stance on long-simmering territorial disputes.

Beijing and Tokyo have scrambled jets in the skies near disputed islands in the East China Sea administered by Tokyo but claimed by China. Beijing is also at odds with Southeast Asian countries over islets in the South China Sea.

"It won't be good news for the world if a large country like China cannot protect its own security," Fu said.

"Strengthening China's defence capability will be conducive to further stability in the region and will be conducive to world peace."

Fu, a vice foreign minister and former ambassador to Britain, added: "China's national defence policy has always been peaceful and defensive.

"We strengthen national defence to defend ourselves and to protect security and peace, but not to pose a threat to other countries," she said.