China's government said on Tuesday that it had asked for a hold on a United States (US) request for United Nations sanctions against ships accused of helping North Korea evade sanctions imposed over its weapons programmes while it looks into who would be affected.

Beijing needs time to investigate because the request to ban violators from global ports and blacklist trading companies covers a “large number of ships and entities,” said a foreign ministry spokesman, Geng Shuang.

Washington requested the sanctions following reports foreign traders are helping North Korea violate limits on oil imports and other trade imposed in an effort to pressure leader Kim Jong Un to stop nuclear and missile development.

China was a long-time diplomatic protector of North Korea and accounts for most of its trade but has supported the latest sanctions out of frustration with Pyongyang's weapons development. Beijing has argued against measures that might destabilise Kim's government.

“It takes some time for us to look into the matter together with other agencies,” said Geng at a regular news briefing.

“We put a hold on the relevant proposal for a technical reason,” he said. “We will make our decision in light of the Security Council resolutions' requirements and on the basis of the fact.”

US and Japanese authorities say they have photographer traders transferring oil at sea to North Korean vessels in violation of the sanctions.

South Korean authorities have seized two vessels suspected of moving oil to the North. Taiwanese authorities are investigating a businessman suspected of arranging oil sales. Australian authorities have arrested a man accused of trying to broker sales of goods including components for ballistic missiles to North Korea.

Last week, the Maldives government denied a report by Japan's foreign ministry that a Maldivian-flagged ship transferred goods to a North Korean-flagged tanker at sea.