UPDATE 5-Australian military to help enforce Sydney COVID-19 lockdown as cases rise

* Sydney records 239 COVID-19 cases in 24 hours

* 300 military personnel to aid police

* Troops to aid enforcement of lockdown

* Sydney lockdown threatens second recession (Adds details of Australian military personnel helping enforce lockdown of Sydney)

By Colin Packham and Renju Jose

SYDNEY, July 29 (Reuters) - Australia's military will help enforce a lockdown in Sydney after the city of 6 million posted a record daily rise in COVID-19 cases on Thursday and state authorities said the outbreak was likely to get worse.

The lockdown of Australia's biggest city has increased pressure on Prime Minister Scott Morrison, who is now trailing in opinion polls, and heightened concern that Australia's A$2 trillion ($1.5 trillion) economy could slide into recession.

Despite an extended lockdown in Sydney since an outbreak of the highly infectious Delta variant, 239 new locally acquired coronavirus cases were recorded in the city over a 24-hour period, the biggest daily rise since the pandemic began.

"We can only assume that things are likely to get worse before they get better given the quantity of people infectious in the community," said Gladys Berejiklian, the premier of New South Wales state, of which Sydney is the capital.

Berejiklian said one new death took the toll from the latest outbreak to 13 and the national total was now 921.

With little sign that of restrictions reducing infections, Berejiklian said new curbs would be imposed on the southwestern and western areas of Sydney where the majority of COVID-19 cases are being found.

Residents there will be forced to wear masks outdoors and to stay within five km (three miles) of their homes.

With even tighter restrictions set to begin on Friday, New South Wales Police said it had asked for 300 military personnel to help enforce lockdown orders.

The personnel will deploy on Friday, Defence Minister Peter Dutton said, and will begin assisting police with ensuring compliance with restrictions next week.

LOCKDOWN EXTENDED

Berejiklian extended the Sydney lockdown by another month on Wednesday.

New South Wales accounts for over a third of Australia's economy. Federal Treasurer Josh Frydenberg said he expected the national economy to shrink in the September quarter but the ability to avoid a technical recession would depend on whether New South Wales can avoid a longer lockdown.

"With respect to the December quarter, that does depend to a large extent how successful New South Wales, our largest state economy, is in getting on top of this virus," Frydenberg told the Australian Broadcasting Corp.

Berejiklian has said restrictions need to remain as too few people in Sydney are vaccinated amid tight supplies of the Pfizer/BioNTech vaccine, with which Australia had hoped to inoculate everyone under 60 years old.

All adults in Sydney have been urged to seek an AstraZeneca vaccine. Some, citing rare blood clots, are reluctant and would prefer to wait several months when Australia is expected to receive additional Pfizer supplies.

($1 = 1.3561 Australian dollars) (Reporting by Renju Jose and Colin Packham in Canberra; additonal reporting by Xihao Jiang in Tokyo; Editing by Michael Perry, Lincoln Feast and Timothy Heritage)