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)