Australia has sealed an agreement with Singapore to post undercover armed guards on flights between the two countries.

It is a significant step forward for air security in the region Australian Justice Minister Chris Ellison

Sky marshals began patrolling Australia's domestic flights after 11 September, but this was the first agreement with a foreign government, said Australian Justice Minister Chris Ellison.

Mr Ellison said the air route to and from Singapore was one of Australia's busiest, with more than 12,000 flights a year between the two countries.

Australia was seeking a similar agreement with Indonesia and the United States, he said.

Fears of terrorist attack have increased in Australia since the October attack on two nightclubs in Bali, which killed 88 Australians.

Improving air safety

Once details are worked out, Mr Ellison said, Australia will begin placing guards on some of the 4,000 flights made to Singapore each year by Qantas.

He said Singapore had not yet decided if it would respond in kind, but Australia would welcome such a step.

Australia already places more than 70 plainclothes officers on domestic flights, and that number will increase to 110 early next year, AP news agency reported.

"The... government's negotiations to expand the (security) programme to include international operations will provide the public with greater confidence and safety in air travel," Mr Ellison said in a statement.

But the idea of sky marshals has been greeted with caution by the industry.

Airlines have voiced concern that having guns on planes could pose more of a risk than the initial threat.