MEXICO CITY (Reuters) - An as yet unknown amount of funds were sucked out of at least five Mexican financial groups through “unauthorized transfers” in recent days, a top central bank official said on Friday, while stopping short of calling it a cyber attack.

Lorenza Martinez, head of Banxico’s payment system, told Reuters that it was still unclear how much money had been fraudulently transferred and she refrained from naming the affected institutions, which could include banks and brokers.

“These unauthorized transfers were originated in the system that connects the institutions to the payment system,” Martinez said in a telephone interview, noting that banks had to migrate to an alternate, slower technology to process payments.

Slow interbank transfers since the end of April and terse statements by authorities have fed concerns in social media that Latin America’s second biggest economy could be the latest victim of cyber attacks that have hit central banks and financial groups around the world.

Martinez said that the central bank’s SPEI interbank transfer system was not compromised but that the problem had to do with software developed by institutions or third-party providers to connect to the payment system.

Mexico’s SPEI system is a domestic network similar to the SWIFT global messaging system that moves trillions of dollars each day.

Hackers have used SWIFT connections to target banks around the world, but the Brussels-based company has not disclosed the number of attacks.

Martinez refrained from calling the incidents in Mexico a cyber attack. “At this time, we cannot reject any hypothesis,” she said. “It was something done on purpose, but how it was done, we are in the process of finding out.”

Martinez said that no clients had been affected since the transfers hit accounts of financial institutions in the central bank.

The funds had been wired to accounts that appeared to be false, she said.

Local banks were carrying out their own investigations with security experts and were in the process of filing charges with authorities after the incident that also involved cash withdrawals from the bogus accounts, she said.

In late April, Mexican bank Banorte reported an “incident” that slowed transactions and Mexico’s central bank said it was investigating the problems that hit at least three local financial institutions.

On Friday, Citibanamex, the local unit of Citigroup Inc, said on Friday that some of its clients had experienced delays in interbank transfers but that it did not have “any problems in its payment systems.”