March 18 (Reuters) - Nasdaq, which hosts some

of the biggest U.S. tech giants on its exchange, said on Monday

it was investigating an issue related to connectivity and stock

orders, marking its second technical glitch in months.

The company did not give details about the severity of the

issue, but said it was related to the matching engine - software

systems that match buy and sell orders on an exchange. The

incident started around 4.55 a.m. ET, according to the

exchange's website.

Thousands of stocks are listed on the Nasdaq, including

those of iPhone maker Apple, electric vehicle-maker

Tesla and artificial intelligence semiconductor

powerhouse Nvidia.

Nasdaq did not immediately respond to a request for comment

outside regular business hours in the U.S., but an updated alert

on its website said it had determined the root cause behind the

issue.

The exchange operator had reported similar issues in

December, when stock orders were impacted and over 50 clients at

the exchange were affected.

Nasdaq said on Monday orders sent using the "RASH FIX"

order handling system were not being acknowledged at this time.

FIX, or Financial Information Exchange, is a message

protocol that defines an electronic message exchange for

communicating securities transactions between two parties.

New York Stock Exchange Arca Equities, which specializes in

listings of exchange-traded products, said it had declared a

self-help alert against Nasdaq.

Cboe also said it "has declared self-help against

another market center, or is actively investigating an issue"

without disclosing the identity of the center.

A "self-help" is a notification issued by a trading

exchange when another exchange is dealing with internal problems

processing trades and orders are routed through alternate

venues.