All three ports in Bahrain remained open on Wednesday, but operations were slow due to growing unrest in the island nation, shipping sources said.

Two Bahraini policemen were killed on Wednesday during an operation to clear mostly Shi'ite protesters from a central roundabout in capital Manama, a health official said.

Bahrain declared martial law on Tuesday, looking to end weeks of protests by its Shi'ite Muslim majority.

"All the ports are open: Sitra, Mina Sulman and Hidd," said a shipping source.

"But there are staff shortages because of the protests, that's why cargo unloading is very slow," the source added.

Bahrain is an exporter of high grade oil products, particularly to Europe and northern Asia.

The large 267,000 barrel per day (bpd) state-run Bahrain Petroleum Co refinery was partially shut for planned maintenance, but the country was still exporting significant volumes of products, traders said.

"Shipments have been running normally and there is no interruption of service. The refinery is undergoing a partial shutdown, but that was planned well in advance and stocks were built up for it," said a Bahrain-based oil trader.