OAKLAND -- Facing yet another deadline to avert a strike, BART and its labor unions on Thursday resumed negotiations for the first time this week as they awaited word from Gov. Jerry Brown on whether they would have two months or three days to reach a deal.

Union officials said in a statement Thursday evening that they had not even discussed the main economic issues of pay and benefits during the bargaining session. They reiterated that they would strike on Monday if they did not reach a deal and Brown did not issue another strike ban before then.

"In the event that negotiations fail, we may issue a 48-hour notice (Friday)," said Josie Mooney, chief negotiator of the local Service Employees International Union.

The bargaining restarted as California's two U.S. senators got involved in the dispute for the first time, sending management and union leaders a brief letter that does not take sides but encourages them to seal a deal to avoid another commuter rail strike.

"We urge you to resume negotiations in good faith, end the dispute, and work together to avoid any further disruptions to BART service," said the letter from Democrats Barbara Boxer and Dianne Feinstein, the highest-ranking federal officials to intervene in the talks.

The seven-day strike ban that Brown imposed last weekend expires Sunday night, and unions have promised to shut down the rail line on Monday for the second time this summer if there's no agreement. The governor is expected to rule in the coming days if he will order a 60-day "cooling-off" period.

With both sides busy preparing their cases to Brown's panel of "fact-finders" this week, the talks at the bargaining table had ceased until negotiators agreed to meet again at a neutral state office in Oakland on Thursday morning. As of Thursday evening, no agreement had been reached, and negotiations were set to resume Friday and possibly through the weekend, though they were as much as $100 million apart.

But Thursday's bargaining was delayed at least an hour after the unions' negotiators showed up to BART's regularly scheduled board of directors meeting, as they have consistently since negotiations began this spring. Though the labor issue was not on the open-session agenda, workers once again blasted the board and the rail line's staff for offering proposals they say shortchange employees and have forced them to walk out.

"We're not being unreasonable. We're not being greedy. We're not being any of those things," said Roxanne Sanchez, president of the local Service Employees International Union. "We're advocates for a standard of living."

As they did during a hearing before Brown's panel on Wednesday, the labor leaders said they could easily reach a deal by Sunday night as long as BART "puts a sufficient amount of money on the table," as George Popyack, director of one of the smaller unions, the local American Federation of State, County and Municipal Employees, told board members.

Board members did not address the labor talks in open session. They then met behind closed doors but did not take any action.

"We gave the general manager the authority and flexibility to reach a fair deal, as long as the unions are ready to move towards a reasonable settlement," Board President Tom Radulovich said in a statement. "We stand ready to compromise with them on contract terms on which all parties can agree."

Contact Mike Rosenberg at 408-920-5705. Follow him at twitter.com/RosenbergMerc.