(Reuters) - Lead U.S. oil company negotiator Shell Oil Co said face-to-face negotiations on Friday with the United Steelworkers Union (USW) failed to yield an agreement to end the 20-day-old U.S. refinery strike.

Workers at several refineries and chemical plants were waiting for instructions to join the more than 5,000 workers at 11 plants, including nine refineries accounting for 13 percent of U.S. production capacity, walking picket lines in the largest national refinery strike since 1980.

Rumors about a tentative deal between the USW and Shell Oil, which is the U.S. arm of Royal Dutch Shell Plc, were being swapped through text messages, emails and phone calls among oil industry insiders throughout the night.

"Discussions with the United Steelworkers concluded for this evening with no agreement in place," said Shell spokesman Ray Fisher.

The Steelworkers said in a message to members and news media, including Reuters, that talks were continuing.

A USW spokeswoman confirmed the text message, but had no further information whether the talks were continuing on Friday night or some future date.

Shell and the union have been meeting continuously since talks resumed on Wednesday following a week-long break for the company to reply to an information request and a counterproposal from the USW.

Union negotiators rejected the seventh contract offer from Shell on Thursday night.

Earlier this week, the USW's lead negotiator, International Vice President Gary Beevers, told Reuters that safe staffing levels at refineries and chemical plants were a sticking point in the talks.

The strike widened on Feb. 6, when workers at two refineries operated by BP Plc were told to walk off their jobs the following day.

In addition to the two BP-operated plants, workers are striking at refineries and plants owned by Lyondell Basell, Marathon Petroleum Corp, Shell, and Tesoro Corp in California, Kentucky, Texas and Washington state.

Only one refinery has shut down due to the strike.

Tesoro Corp's 166,000-bpd plant in Martinez, California was scheduled prior to the strike for a partial shutdown to perform a planned multi-unit overhaul. Company officials decided to idle the entire plant after the walkout began and said production would not resume for the duration of the work stoppage.

The USW is seeking a three-year, industry wide pact that would cover 30,000 workers at 63 U.S. refineries that together account for two-thirds of domestic capacity.

Companies have called on temporary replacement workers to keep plants running at nearly normal levels.