SYDNEY (Reuters) - Australia will pay an extra $300 million to help develop Afghanistan's security forces, the government said on Friday ahead of a NATO leaders' summit in Warsaw, where tenuous security in Afghanistan will be a central discussion topic.

Australia, a firm U.S. ally in its wars in Afghanistan and Iraq, had originally pledged $500 million from 2010-2017 but will now provide an extra $100 million annually until 2020.

The announcement of extra funding came two days after U.S. President Barack Obama said he had decided to postpone plans to cut the number of U.S. forces in Afghanistan almost in half by the end of the year, instead keeping 8,400 troops there from January.

There are also another 3,000 international troops serving in the NATO-led mission in Afghanistan.

Australian Prime Minister Malcolm Turnbull visited Kabul in January, and the Australian Defence Force announced two months later its troops would remain there for at least another three years. About 270 Australian troops are serving in the NATO-led force.

No senior Australian government members will attend the Warsaw summit, with a cliffhanger nation election a week ago still without a clear result.

The $100 million annual commitment comprises an $80 million contribution to the Afghan National Army and $20 million for the Afghan police.

(Reporting by Tom Westbrook; Editing by Paul Tait)