Union leaders urged striking Los Angeles teachers to turn out en masse for a rally on Friday, marking the fifth day of their walkout against the US's second-largest school district and seeking to boost their bargaining position in marathon labour talks.

Some 30,000 teachers who have gone without a contract for nearly a year walked off the job on Monday demanding higher pay, smaller classes and more support staff. It was the first such work stoppage to hit the Los Angeles Unified School District (LAUSD) in nearly three decades.

The labour strife in Los Angeles follows a wave of teacher strikes last year across the United States over salaries and school funding, including walkouts in West Virginia, Oklahoma and Arizona.

Another key point of contention in Los Angeles has been the teachers' calls for restraint in LAUSD's steady expansion of independently managed charter schools that the union argues are diverting resources from traditional classroom instruction.

School District Superintendent Austin Beutner has said the demands, if fully met, would inflict too great a budget strain. The union's president, Alex Caputo-Pearl, has insisted sufficient funding is available given the right priorities.

The two sides returned to the bargaining table around midday on Thursday, meeting for the first time since 21 months of talks broke down a week ago with the union's rejection of what was then management's latest offer.

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Mayor Eric Garcetti, who has no direct authority over the school district, was acting as a mediator in the talks.

A union spokeswoman told Reuters news agency by email Thursday night that negotiators were still at it some nine hours after they started. It was not clear how long they would continue to work before taking a break.

Second-grade teacher Robin Longino told the Los Angeles Times daily that she hoped for an agreement soon. "We're just ready for this to be over," Longino said.

"This is the result of neglect for the last 30 years," she said.

'Not going to be months'

At a news conference earlier in the evening, Caputo-Pearl said bargaining would likely extend into the weekend. "An agreement is not going to take shape overnight," he said, but added, "This is not going to be months."

A district spokeswoman, Shannon Haber, declined comment.

Caputo-Pearl said it was crucial for union rank-and-file to achieve a major turnout at a rally planned for Friday morning in downtown Grand Park.

"The more power that we show with our numbers in Grand Park, the more power our bargaining team has in bargaining," he said. "It's that simple."

Forecasts call for sunny skies on Friday after drenching downpours that marked the strike's first four days.

The strike has disrupted classes for nearly 500,000 students, though support for teachers was running high among parents and among the public at large as reflected in a recent survey of Los Angeles residents.

The walkout also has drawn gestures of solidarity from several major politicians considered likely contenders for the Democratic 2020 presidential nomination, including Garcetti and US Senators Bernie Sanders, Elizabeth Warren and Kamala Harris.

District officials have kept all 1,200 schools open on a limited basis with a skeleton staff, but attendance was running well below normal.

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Caputo-Pearl cited teachers' demands to reduce class size by hiring more teachers as "arguably the most fundamental" stumbling block. The parties are closer on salary.

The union wants a 6.5 percent pay rise. The district has offered six percent with back pay. LAUSD teacher pay currently averages $75,000, state figures show.

Caputo-Pearl said California Governor Gavin Newsom has weighed in with both sides and would likely "play a key role" in helping clinch a deal.

Speaking to the Los Angeles Times, second-grade teacher Yosana Joaquin said she believes a deal is near. "I feel like we're going to be back on Tuesday," she told the paper.