•US regulators called the move "most welcome", while the banks said their action reflected their "confidence".
•The San Francisco-based firm had seen its share price plunge nearly 70% over the last week, as investors worried it was the next bank at risk of a rush of customers withdrawing their deposits.
A group of big US banks has injected $30bn (£24.8bn) into a smaller regional bank, First Republic, which had been seen as at risk of failure.
The move came as authorities in the US are trying to quell panic over the health of the banking system, after a series of bank collapses in the US.
Worries about the sector have spread globally, raising fears of a crisis.
US regulators called the move "most welcome", while the banks said their action reflected their "confidence".
They said the banking system had plenty of cash and made big profits.
"Recent events did nothing to change this," they said. "The actions of America's largest banks reflect their confidence in the country's banking system."
Reports of plans for the aid from the 11 banks, led by JP Morgan and Citigroup, helped lift financial markets and sent shares in First Republic surging more than 20% at one point, triggering trading halts.
But a sell-off started again in after-hours trade in a sign that concerns remain.
The San Francisco-based firm had seen its share price plunge nearly 70% over the last week, as investors worried it was the next bank at risk of a rush of customers withdrawing their deposits.
"This show of support by a group of large banks is most welcome, and demonstrates the resilience of the banking system," US financial officials said.
'Risk of contagion'
Problems in the banking sector surfaced in the US last week when Silicon Valley Bank (SVB), the country's 16th-largest lender, collapsed in the biggest failure of a US bank since 2008.
That was followed two days later by the failure of New York's Signature Bank.
Authorities stepped in to guarantee deposits beyond typical limits in an effort to head off further runs on bank deposits, but financial markets have remained jumpy.
In a sign of strains in the system, the US central bank reported a surge in emergency lending to banks, with $318bn in outstanding loans as of Wednesday, up from $15bn a week earlier.
That included roughly $12bn offered through a fund created after the SVB collapse.
"The size of the spike in the Fed's emergency lending underlines that this is a very serious crisis in the banking system that will have significant knock-on effects on the real economy," Paul Ashworth, chief North America economist at Capital Economics said.
In an appearance before politicians in Washington, US Treasury Secretary Janet Yellen said that depositors should have confidence in the system, while acknowledging the severity of the episode.
"We felt that there was serious risk of contagion that could have brought down and triggered runs on many banks and that's something, given that our judgement is that the banking system overall is safe and sound," she said.
Meanwhile, the vice president of the European Central Bank (ECB), Luis de Guindos, said the banking industry in Europe was "resilient" and firms there had "limited exposure to the institutions of the US".
He spoke as the ECB announced a further increase to interest rates from 2.5% to 3%, sticking to its plan for a rise despite concerns about how the move might affect the market turmoil.
Help for banks
Central banks around the world have sharply raised borrowing costs over the last year to try to curb the pace of overall price rises, or inflation.
The moves have hurt the values of the large portfolios of bonds bought by banks when rates were lower, a change that contributed to the collapse of Silicon Valley Bank, and has raised questions about the situation at other firms.
The Swiss National Bank on Wednesday said it was extending up to £44bn in emergency funds to troubled lending giant Credit Suisse, which was seen as vulnerable in the wake of the US bank failures.
Its shares bounced back more than 15% after big falls a day earlier, while major indexes across Europe also gained.
Sir John Gieve, former deputy governor at the Bank of England, told the BBC that central banks were sending a "message" that such problems would be contained locally.
He added that in Credit Suisse's case, the Swiss National Bank's action was likely to be enough to stop the crisis spreading.
"What we've seen overnight is the Swiss central bank saying 'no, we will not let this get into a disorderly collapse'," he said.
"I don't know what the future for Credit Suisse holds but so far they are still standing and it looks like the Swiss central bank will ensure it's standing long enough to rearrange its affairs for the future."
Credit Suisse, founded in 1856, has faced a string of scandals in recent years, including money laundering charges, spying allegations and high profile departures.
It lost money in 2021 and again in 2022 and has warned it does not expect to be profitable until next year. Customers pulled millions of dollars from the firm in recent months.
White House spokesperson Karine Jean-Pierre said officials had been monitoring the developments at Credit Suisse but its troubles were "distinct" from events in the US.
"Its problems are not related to the current economic situation," she said.