More than 1,600 social workers in Los Angeles county have gone on strike for better pay and fewer caseloads, saying excessive demands were buckling the system and jeopardising children's safety.

Workers from the department of children and family services were expected to resume picketing offices on Friday a day after launching the first strike of its kind in over a decade.

The 721 local branch of the Service Employees International Union called the action a last resort after talks broke down on Wednesday night, saying its members were snowed under with caseloads and unable to do their jobs properly.

“We've made it clear that we're serious about change and we will not stop until LA county commits in writing to safer social worker-to-child ratios,” the union declared on its website.

Pickets would continue at more than a dozen locations and strikers would demonstrate in front of the department's headquarters, it said.

"We can't make the right decisions when we don't see our families enough, we're so overwhelmed by paperwork and all kinds of other things that cases involve," Delmi Madrigal told radio station KPCC. "We can't do it, we're not going to make the right decisions in that kind of environment."

In New York City social workers have about nine investigations into child abuse open at any one time versus 19 in Los Angeles.

The county chief executive, William Fujioka, said the department was disappointed at the strike and that it would affect a very vulnerable population. The county has drafted hundreds of administrators and managers to plug some of the gaps. Armand Montiel, a department spokesman, said key functions such as child abuse hotlines and emergency response units would work as normal.

The timing of a 6% pay rise was one of the key sticking points but the union focused public statements on caseloads to win favour with private sector workers who have yet to recover from the recession, according to the Los Angeles Times.

“You have to start pleading a case that you feel is going to resonate with the public more than protecting pensions, more than getting raises,” said Jaime Regalado, a political science professor at Cal State LA.

There were no immediate plans for renewed talks between both sides. Zev Yaroslavsky, the county supervisor, ruled out including staffing requirements in a collective bargaining agreement. He accused the union of holding colleagues hostage to demands which would not be realised.