CORRECTED-UPDATE 2-Australia says it will join U.S.-led defence effort in Strait of Hormuz

(Corrects to show Iran has denied being responsible for attacks on ships, paragraph 2)

* Australia agrees to U.S. request to join U.S.-led coalition

* Coalition guarding shipments through strategic waterway

* Australia says deployment is 'modest' and 'time-limited'

* Almost 20% of oil shipments pass through Strait of Hormuz

By Colin Packham

SYDNEY, Aug 21 (Reuters) - Australia will join the United States in a coalition of countries protecting oil tankers and cargo ships from threats posed by Iran in the Strait of Hormuz, Prime Minister Scott Morrison said on Wednesday.

Global commodity trading has been rocked in recent months after a series of attacks on international merchant vessels, which the United States has blamed on Iran, and the seizure of a British tanker. Tehran has denied the accusations.

"This destabilising behaviour is a threat to Australia's interests in the region," Morrison told a news conference in Canberra.

"The government has decided that it is in Australia's national interest to work with our international partners to contribute. Our contribution will be limited in scope and it will be time-bound," he said.

Morrison said Australia will send a P-8A Poseidon surveillance plane to the Middle East for one month before the end of 2019, while an Australian frigate will be deployed in January 2020 for six months.

Australia's deployment will expand U.S-led efforts to secure the strait, which lies between Oman and Iran, after tensions spiked between Iran and the West.

Washington, which has by far the strongest Western naval contingent in the Gulf, has been calling for its allies to join it in an operation to guard shipping.

However, European countries, which disagree with a U.S. decision to impose sanctions on Iran, have been reluctant to sign up to a U.S.-led mission for fear of adding to tension in the region.

U.S. President Donald Trump withdrew Washington in 2018 from an international deal to curb Iran's nuclear ambitions.

Trump then applied widespread sanctions in May designed to block all of Iran's oil exports, the lifeblood of Tehran's economy. Iranian officials denounced the new penalties as "economic warfare".

Canberra, although keen to avoid stoking tensions, was widely expected to join the U.S.-led maritime coalition.

Australia is a staunch ally of the United States, which in recent months has called on its partners to do more to defend global security. (Reporting by Colin Packham in Sydney Additional reporting by Wayne Cole in Sydney Editing by Paul Tait)