* Authorities said to have intervened last week to

support currency

* Japan closed for Golden Week holiday

* US to aid ships stranded behind Strait of Hormuz

(Updates after sudden rise in yen)

By Gregor Stuart Hunter

SINGAPORE, May 4 (Reuters) - The yen strengthened

suddenly against the dollar on Monday with markets on alert for

action by authorities following suspected intervention by them

last week to bolster the battered currency.

The yen climbed as much as 0.75% to 155.69 before paring

gains, with much of the appreciation coming during a nine-minute

stretch around midday Singapore time.

"It could be them again," said Nick Twidale, chief market

strategist at ATFX Global in Sydney. "Certainly not to the same

extent as last week, but reinforcing their stance that they

won't accept a weak yen."

Ministry of Finance officials did not immediately respond to

requests for comment.

Tokyo officials declined to confirm whether they had

intervened last week, but sources told Reuters the authorities

did undertake yen-buying activity for the first time in two

years.

Analysts have questioned whether unilateral intervention,

the third such effort in the past four years, would prove

effective.

"The primary focus will be whether further intervention

occurs, noting that Japan is closed for the Golden Week holiday

and there will be thinner liquidity during this time," said

Mahjabeen Zaman, head of FX research at ANZ Bank in Sydney.

"And more importantly, whether the U.S. will join Japan's

efforts in supporting the yen," she added. "If the yen weakens

further, you could argue that the likelihood of bilateral

intervention increases."

Markets remained cautious at the start of trading after

President Donald Trump said the U.S. would start an effort on

Monday morning to free ships stranded in the Strait of Hormuz as

a "humanitarian gesture" to aid neutral countries in the

U.S.-Israeli war with Iran.

The U.S. dollar index, which measures the greenback's

strength against a basket of six currencies, was down 0.1% at

98.041.

The Australian dollar was up 0.2% at $0.7214, while

its kiwi counterpart advanced 0.5% at $0.5922.

The Reserve Bank of Australia is due to announce its next policy

decision on Tuesday, with the majority of analysts polled by

Reuters expecting a hike in the cash rate to 4.35%. Last week,

Australia's top two grocers warned of growing price pressures as

the Iran war drives up fuel and raw material costs for

suppliers.

The euro was up 0.1% at $1.1730 after German Chancellor

Friedrich Merz sought to downplay a rift with Trump after a

planned troop drawdown was announced. The country's economy

ministry said on Sunday that Berlin is also in touch with the

European Commission as it holds talks with Washington, after

Trump said on Friday he would increase tariffs on cars and

trucks from the EU to 25%.

The British pound was up 0.1% at $1.3595.

Bitcoin was up 1.8% at $80,337.53, while ether

advanced 2.4% to $2,384.51.

(Reporting by Gregor Stuart Hunter; Editing by Muralikumar

Anantharaman)