The San Francisco Employees’ Retirement System’s governing board took tentative steps toward selling some of its $559 million stake in fossil fuel companies Wednesday, agreeing to instruct its staff to begin assessing how the fund will define its “riskiest and dirtiest” investments.

The pension plan’s staff will have until April 30 to come up with a plan to assess those investments. Once the board is satisfied with that plan, the staff instructed to plan on how to sell off those assets, reporting back in the late summer or early fall.

“I want benchmarks and clarity,” said Malia Cohen, a city supervisor who also serves on the SFERS board.

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Citing the environmental risks posed by the fossil fuel industry, political and environmental activists have called on the SFERS board to pull out of its investments in oil, natural gas and coal companies and embrace emerging renewable energy technologies. The SFERS staff has long advised the board against doing so, however, citing the potential financial damage to the fund.

The board’s moves Wednesday proved deeply frustrating for many in attendance Wednesday, who had sought more significant steps toward divestment.

“It just feels so vague and non-committal,” said Jack Fleck, a retiree who worked for the city for 25 years.

Victor Makras, a SFERS board member and strong divestment proponent, said he remained optimistic that the board would “vote on the first round stocks to divest” by August, he said.

The board also agreed to shift $1 billion in investments away from high-emission polluters and to hire a new staff member to help ensure the pension fund is engaging in “socially responsible investing.”

Dominic Fracassa is a San Francisco Chronicle staff writer. Email: dfracassa@sfchronicle.com Twitter: @dominicfracassa