LONDON (Reuters) - The safety risks posed by unregulated oil tankers are rising, and the so-called shadow fleet is a threat to both the maritime environment and seafarers, the head of the United Nations' shipping agency said on Tuesday.

The shadow fleet refers to hundreds of ships used by Russia to move oil, in violation of international restrictions imposed on it over the Ukraine war, as well as by oil exporters such as Iran and Venezuela hit by U.S. sanctions.

At least 65 oil tankers dropped anchor this week at multiple locations, including off the coasts of China and Russia, since the United States announced a new sanctions package on Jan. 10.

"The risk is growing in relation to the environmental impact and the safety of the seafarers as the shadow fleet grows," Arsenio Dominguez, Secretary-General of the International Maritime Organization (IMO), told a news conference.

"We see it by different accidents and events that have taken place."

Dominguez, who could not comment on sanctions, said his biggest concern was with ageing tankers, which were "putting people onboard at risk and the environment as well".

"The more that ships start looking to ... avoid meeting the IMO requirements, the more that we will have situations like we have been experiencing in the last part of 2024."

There have been a number of incidents involving collisions and shadow fleet vessels breaking down in recent months.

Dominguez said an IMO meeting would follow up in March on a resolution adopted in 2023 aimed at greater scrutiny of ship-to-ship oil transfers in open seas - a frequent risk with shadow fleet tankers which carry out such transfers with little regard for safety.

He said he had also met with smaller flag registry countries, which typically provide flagging for shadow fleet tankers.

Commercial ships must be registered, or flagged, with a particular country to ensure they are complying with internationally recognised safety and environmental rules.

Shipping industry sources say many of the smaller flag registries are lax about enforcing compliance and also sanctions regulations.

"Substandard shipping ...has been on the agenda at IMO for many years," Dominguez said.

(Reporting by Jonathan Saul; Editing by Ros Russell)