BART station agent Mark Madrigal helps a customer at the Lake Merritt station in Oakland, Calif., Friday, June 28, 2013. Two of San Francisco Bay Area Rapid Transit's largest unions gave notice that they plan to go on strike if they don't reach a new contract deal over the weekend. Union leader Dwight McElroy, center, conducts interview alongside fellow union members in Oakland, Calif., Friday, June 28, 2013. (AP) - A chief negotiator said Saturday that two of San Francisco Bay Area Rapid Transit's largest unions will "likely" go on strike. Josie Mooney, a negotiator for the Service Employees International Union (SEIU), Local 1021 SEIU, said there's "a 95 percent chance" that her union and the Amalgamated Transit Union (ATU), Local 1555, will be on strike Monday after their contracts expire late Sunday. "I'm afraid I don't see a way we will avoid a strike," she said after union leaders left Saturday's negotiations claiming they have met with BART's management for only 10 minutes in the past 36 hours. The walkout adds further speculation that a strike could derail the more than 400,000 riders who use the nation's fifth-largest rail system. Mooney said the unions have no plans to meet with BART on Sunday. BART spokesman Rick Rice said Saturday says that the agency still has a meeting scheduled with the unions on Sunday. He said BART has submitted its second new proposal to the unions since Thursday. "We called our mediator to deliver it to them and informed us they had left the building," Rice said. Maybe they will show a willingness to come back to the table. We'll still be here. With a Sunday night deadline approaching, negotiations between BART and the unions had intensified with a possible strike at stake. As the parties went back to the bargaining table Saturday in Oakland for anticipated around-the-clock sessions, both sides described the talks as tense and said they were far apart on key sticking points including salary, pensions, health care and safety. A work stoppage that could start as early as Monday would be chaotic for commuters and affect every mode of transportation, clogging highways and bridges throughout the Bay Area. More than 400,000 riders use BART each day. The unions want a 5 percent annual raise over the next three years. BART said Saturday that train operators and station agents in the unions average about $71,000 in base salary and $11,000 in overtime annually. The workers also pay a flat $92 monthly fee for health insurance. Meanwhile, BART has offered a 1 percent raise annually over the next four years and for employees to contribute to their pensions. The unions' current contract expires at midnight Sunday. On Friday, the ATU asked California Gov. Jerry Brown to issue a 60-day "cooling off" period if no deal can be reached by Sunday's deadline, but the SEIU and BART officials have urged Brown not to issue such an order. The governor's office has declined to comment. "Negotiations are frustrating," Rice said. But, we'll be here, no matter long it takes. We're committed to work this out. BART's last strike lasted six days in 1997. On Friday, other area transit agencies urged commuters to consider carpooling, taking buses or ferries, working from home and, if they must drive to work, to leave earlier or even later than usual. "The bottom line is that a BART strike will be an absolute nightmare for everyone," said Jim Wunderman, president and CEO of the Bay Area Council, a business advocacy organization. Our transportation system simply does not have the capacity to absorb the more than 400,000 BART riders who will be left at the station. There will be serious pain.