The United States sees Iran's capacity to move its oil as reliant on service providers based in Malaysia, with oil being transferred near Singapore, the US Treasury Department's top sanctions official said on Tuesday.

Brian Nelson, Treasury's undersecretary for terrorism and financial intelligence, was speaking during a four-day visit to Singapore and Malaysia, which the department said aimed to advance its work in countering financing and revenue generation by Iran and its proxies.

The trip comes as Treasury increases its focus on financing for militant groups routed through Southeast Asia, including through fundraising efforts and illicit sales of Iranian oil.

Nelson told reporters the United States was trying to prevent Malaysia from becoming a jurisdiction where the Palestinian militant group Hamas could both fundraise and then move money.

He said the United States saw Iranian oil being transferred near Singapore and throughout the region.

Last December, Treasury imposed sanctions on four Malaysia-based companies it accused of being fronts supporting Iran's production of drones.

Nelson also said sanctions and export controls against Russia were seeing progress, saying the Russian oil price cap was reducing Moscow's capacity to profit from oil sales while preserving the stability of global energy markets.

Singapore is a major shipping hub. Insurance and other maritime service providers operating in Singapore have warned of evasion of the price cap on Russian oil, complaining that it is difficult to confirm that paperwork promising oil is bought at or below the $60 cap is accurate.

(Reporting by Reuters)