(This March 16 story has been refiled to correct a word in the quote in paragraph 7)

By Lananh Nguyen, Pete Schroeder, Andrea Shalal and Megan Davies

(Reuters) - First Republic Bank's shares fell 17% in extended trading on Thursday, despite an unprecedented show of support in the bank from nearly a dozen of the world's largest financial institutions.

In an unusual rescue that several sources said was orchestrated by JPMorgan Chase & Co Chief Executive Jamie Dimon earlier this week along with Treasury Secretary Janet Yellen and Federal Reserve Chair Jerome Powell, 11 Wall Street firms said they were depositing $30 billion into First Republic.

Investors' relief, however, was short-lived. The bank's shares, which had closed 10% higher after a volatile day that saw trading halted 17 times, slumped in after-market trading. Volume hit 15.6 million shares in the post-market session.

The reversal came after First Republic said in a filing that it was suspending its dividend. It also said it had a cash position of about $34 billion, excluding the $30 billion in new deposits.

The company also said it had borrowed up to $109 billion from the Fed between March 10 and March 15, and an additional $10 billion from the Federal Home Loan Bank on March 9.

The reversal in First Republic's shares after the rescue deal from the biggest U.S. banks underscores the extent of jitters in global markets, set in motion when two regional banks failed. Separate attempts earlier this week by U.S. and European regulators to calm investors through emergency measures to shore up confidence in the banking sector have not stuck.

Jason Ware, chief investment officer for Albion Financial Group, said the Dimon-led banking sector intervention on Thursday was a "shot in the arm for the system" but likely more was needed. "It's not big enough," Ware said.

Ware added that it also crystallized in investors' minds that there were deeper problems at First Republic.

RESCUE PACKAGE

Founded in 1985, First Republic had $212 billion in assets and $176.4 billion in deposits as of the end of last year, according to its annual report.

About 70% of its deposits are uninsured, above the median of 55% for medium-sized banks and the third highest in the group after Silicon Valley Bank and Signature Bank, according to a Bank of America note.

The bank's shares have been hit hard in recent days in the aftermath of the collapse of Silicon Valley Bank.

As the situation got worse, Dimon discussed the idea of a rescue package with Yellen and Powell earlier this week, two sources familiar with the matter said.