International pressure on Israel mounted as the probe into the alleged murder of a top Hamas leader in Dubai reached American and Australian shores.

On Thursday, Australian Foreign Minister Stephen Smith summoned Israel's ambassador and demanded his help in investigating how three Australian passports ended up linked to the death. That followed Dubai's disclosure Wednesday that it had identified 15 more passports, including the Australian documents, carried by suspects in the case, bringing the total number to 26.

"I made it crystal clear to the ambassador that the Australian government regards this as a matter of the gravest concern," Mr. Smith told reporters.

No evidence has yet emerged linking Israel to the death of Mahmoud al-Mabhouh, whose body was found in a Dubai hotel room on Jan. 20. Israeli officials have refused to either confirm or deny any role in the death, a longstanding practice. Earlier this week, Israel's foreign minister said there was no evidence linking Israel with the death.

But the operation immediately triggered accusations of involvement by Israel's Mossad intelligence agency. Shortly after Mr. Mabhouh's body was found, Israeli officials linked him to weapons smuggling between Iran and the Hamas-controlled Gaza Strip. The Hamas commander has long been wanted by Israel for allegedly planning the kidnapping and killing of two Israeli soldiers in 1989.

The United Arab Emirates has also asked for U.S. help in the probe, according to a person familiar with the situation. Dubai police said they have identified credit card accounts used by 14 of the suspects. Most of the cards were issued and distributed by an Iowa-based bank and a New York-based debit-card payments company, with offices outside Tel Aviv, Dubai police said.

The U.S.-issued cards, along with a handful of others issued by European banks, were used by suspects to pay for air travel and hotels, according to Dubai police.

The request could put Washington in an uncomfortable spot. Israel is Washington's closest and strongest Mideast ally. But the U.A.E. is also a crucial American friend, one with growing regional heft and influence.

For years, U.S. government officials have flown into the U.A.E. and other Persian Gulf states, asking for assistance in their own terror-financing probes. The Mabhouh murder case appears to be the first, high-profile investigation in which the roles are reversed: The U.A.E. is now expecting cooperation from Washington in tracking down suspected terror financing through the U.S. banking system.

A spokesman for the U.S. Embassy in Abu Dhabi declined to comment. A spokesman for the U.S. Federal Bureau of Investigation in Washington said, "we just don't comment on these matters."

At a media briefing Thursday, State Department spokesman P.J. Crowley said neither Dubai nor any European country had asked the U.S. for assistance, but "law enforcement would be cooperative if there's anything that we can do or if we come across any information that we think is useful to the investigation."

Early last week, Dubai authorities released passport details of 11 initial suspects, all Europeans, who they were seeking in the case. That triggered a diplomatic furor in Europe. Most of the passport holders quickly emerged, appearing to be victims of identity fraud.

European governments demanded Israel disclose information it might have about how the British, French, German and Irish documents were allegedly obtained. Dubai's police chief, meanwhile, turned up the heat on Israel, saying last week he was "99%" certain Mossad was behind the killing.

The furor comes just as the Mossad's reputation was on the mend. It is believed responsible for a series of other recent operations, which experts credit with enhancing Israeli national security, including the 2008 car bombing of Hezbollah operations chief Imad Mughniyeh.

Mr. Mabhouh's killing has won praise inside Israel. Earlier this week, opposition leader Tzipi Livni called the assassination "good news."

But Israel's international standing has suffered. As the furor over fraudulent passports grows, some analysts are starting to question whether Israel, if it was involved, made a strategic blunder despite a tactical victory.

"When you deter militarily or through means of intelligence methods, you sometimes pay in the diplomatic arena," said Alon Liel, the former director general of Israel's foreign ministry.

Meanwhile, the growing number of Israeli residents identified as holding passports linked to the case by Dubai authorities is drawing fresh suspicion from outside. So far, at least 13 of the 26 European and Australian passport holders identified in the probe are Israeli residents.

Philip Carr, a 36-year-old British native who resides in the suburb of Bet Shemesh, said he was surprised to find his name on Dubai's suspect list.

"I'm surprised and shocked. In 24 hours it still hasn't quite sunk in," he said. "I don't know what else to say except that I'm probably somewhat wanted."

—Rachel Pannett in Canberra contributed to this article.