Turkish port authorities have begun informally requiring shipping agents to provide letters declaring that vessels are not linked to Israel and not carrying military or hazardous cargo bound for the country, according to two shipping sources, Reuters reports.

The sources said the harbor master's office had verbally instructed port agents to provide written assurances, adding that there was no official circular on the issue.

One of the sources said the instruction applied to ports across Turkey.

The second source said vessels arriving directly from Israel or departing to Israeli ports would no longer be permitted to dock at Turkish ports.

Turkish-flagged ships will also be prohibited from calling at Israeli ports, according to the source.

The new guarantee letters should state that vessel owners, managers, and operators have no ties to Israel, and that certain types of cargo, including explosives and radioactive materials or military equipment, are not on board en route to Israel, the second source said.

The move is another step Turkey has taken against Israel after it last year severed trade with the country, worth $7 billion annually, over its war in Gaza with Palestinian militant group Hamas.