* Container ships worst affected by the stoppage
* Reason for strike, planned duration not clear
* Potential for delay to coffee, sugar shipments
* Liquids including oil derivatives unaffected - port (Adds details from port spokeswoman)
SAO PAULO, May 30 (Reuters) - Stevedores at Brazil's largest port, Santos, went on strike on Wednesday, threatening delays to shipments of goods including coffee, sugar and other commodities, the port authority said.
The stoppage was in protest at a move by government officials to enforce rules that require stevedores to have a gap of 11 hours between shifts. Stevedores have rejected the idea, Santos port spokeswoman Celia Souza said.
No representatives from the stevedores' association, Sindaport, could be reached Wednesday afternoon to confirm the reason for the strike and how long it might last.
Two-thirds of the ships berthed could not be loaded or unloaded due to the stoppage, Souza said. Those taking on liquid loads such as orange juice and derivatives of crude oil were able to load as usual.
The port spokeswoman said five ships loading bulk cargoes were still taking on goods such as soybeans, sugar, pulp and other commodities, despite the strike.
The flow of container goods is mostly paralyzed due to the required use of stevedores.
About 38 ships were anchored offshore, queueing to berth.
Brazil is the world's No. 1 producer of coffee and sugar and the harvest for both is in its early stages. Raw sugar is exported in bulk, while white sugar and coffee is largely bagged up and packed into containers. The soy harvest has just finished, making this one of the peak flow periods for the crop.
In the past, the stevedores' association at Santos has tended to stage 24-hour warning strikes and threatened to proceed to unlimited stoppages unless their grievances were addressed in negotiations.
Santos is a key exit point for the country's most valuable farm commodities such as soy, sugar and coffee. (Reporting by Roberto Samora; Writing by Peter Murphy; Editing by Reese Ewing,; Kenneth Barry and John Wallace)