Merrill Lynch

( MER) said Thursday it was experiencing "intermittent" outages in the system its brokers use to place orders for retail clients in the U.S.

The firm said its clients should not be affected by the problems, since it intends to execute buy and sell orders at the prices that existed when they were made.

Merrill, the world's biggest brokerage firm, began seeing outages in its financial consultant's desktop order entry system Wednesday afternoon, and the problem grew worse Thursday, said Bobbie Collins, a Merrill spokeswoman. The firm has not yet identified the cause of the system's difficulties, she said. "We're not sure where it's coming from."

Merrill is not promising that the system will be fixed when trading begins again Friday. "We are currently working on the problem," Collins said. "We'll work on the problem through the night until it is fixed."

In the meantime, customer orders were being taken manually throughout the trading day Thursday and processed through a backup system. Because the orders are time-stamped, the orders are going through at the price at which customers placed them, Collins said.

The system's problem is limited to retail clients who use financial consultants to place trades; institutional clients and Internet clients were not affected by the technical difficulties, said Collins, who acknowledged that the firm has not experienced problems like this before.

It is still unclear how much making good on client trades will cost the firm. "At this point, we don't even know how many trades were affected," Collins said.