HOUSTON (Reuters) - Union negotiators rejected the latest offer from oil companies covering workers at 63 U.S. refineries on Saturday night, just hours before a strike deadline, according to a message sent to union members.

The United Steelworkers union (USW) said in the text message sent to members that the latest offer was "insulting and fails to address issues that matter to members."

This is the fourth contract proposal rejected by the USW since negotiations for a new three-year agreement began on Jan. 21. It comes just hours before the contract is set to expire at 12:01 a.m. on Sunday.

The Steelworkers issued strike notices on Friday night to several U.S. refineries where contracts expire on Sunday. While the current national agreement expires on Sunday, each refinery has an individual contract based on the national agreement.

A strike notice is legal notification of a possible strike by a labor union at that location. It does not mean that a strike will take place at that location.

"All of the locals that have contracts expiring on Feb. 1 were instructed to issue strike notices to the companies," one of the sources said.

The sources did not know which refineries received the notices, but only those plants where the local contract expires at 12:01 a.m. local time on Sunday in the time zone where each refinery is located.

Exxon Mobil Corp said its refinery in Beaumont, Texas, received a strike notice.

USW International Vice President Gary Beevers, who is negotiating on behalf of the union, will decide at which refineries strikes will take place after the contract expires.

Negotiators for the USW and Royal Dutch Shell Plc, which is leading the talks on behalf of U.S. refinery owners, were meeting on Saturday to try and reach an agreement before the current national agreement expires.

At least three contract offers have been rejected by USW negotiators since contract talks began on Jan. 21. The most recent rejection was on Friday.

The USW is seeking annual pay raises double those of the last agreement. It also wants work that has been given in the past to non-union contractors to start going to USW members, a tighter policy to prevent workplace fatigue, and reductions in members' out-of-pocket payments for healthcare.

The three-year national contract covers about 30,000 hourly workers at 63 U.S. refineries that account for 64 percent of national refining capacity.