Several major Japanese banks are considering evacuating staff from the Middle East and Wall Street giant JPMorgan has restricted employee travel in the region as tensions escalate.

The United States entered the war against Iran over the weekend with strikes against three key nuclear sites, joining Israel's military campaign to cripple Tehran's nuclear capabilities. Iran has vowed to retaliate.

On Monday, Israel struck Evin prison in northern Tehran, a potent symbol of Iran's governing system, in what Israel called its most intense bombing yet of the Iranian capital.

The war threatens years of effort by Middle Eastern governments to court global financial firms as part of plans to diversify their economies away from oil.

Countries such as Saudi Arabia and the United Arab Emirates have pushed to position themselves as regional financial hubs, offering incentives and regulatory reforms to attract banks and asset managers.

Japan's Sumitomo Mitsui Financial Group has begun evacuating staff from locations including Iran and Qatar to ensure their safety, a spokesperson said.

U.S.-based JPMorgan is allowing only essential travel in and out of the Middle East for employees, a person familiar with the matter told Reuters on Monday.

The largest U.S. bank is offering support to employees on an individual basis, as needed, the source added, requesting anonymity discussing confidential information.

Goldman Sachs had asked its staff in Israel to work remotely about a week ago, according to a source familiar with the matter.

Japan's Mitsubishi UFJ Financial Group has begun evacuating some family members of staff from Dubai and the Saudi Arabian capital Riyadh, a spokesperson said, and is also considering allowing staff to leave at their own discretion.

Ang Wee Khoon, head of risk management at the DIFC branch of Bank of Singapore, one of Asia's biggest private banks, told Reuters the bank has halted all non-essential travel to and from Dubai.

"The safety of our staff is our highest priority, and we stand ready to activate our business continuity plans while minimising the disruptions to clients," Ang said.

Mizuho Financial Group is urging its staff to be cautious and is considering measures including evacuations, a spokesperson said.

(Reporting by Miho Uranaka, Makiko Yamazaki and Sam Nussey in Tokyo, Pritam Biswas in Bengaluru and Yantoultra Ngui in Singapore, Saeed Azhar in New York; Writing by Manya Saini; Editing by Kate Mayberry and Devika Syamnath)