A ship which arrived in the US territory of Guam was turned away by authorities after thousands of spiders poured out of its cargo.

The Guam Department of Agriculture said hundreds of large spiders and thousands of smaller ones were seen when stevedores began offloading insulation and beams for housing units from the ship, the MV Altavia.

The cargo was returned to the ship, and the Agriculture Department ordered that the ship not be allowed to dock. It was last ported in South Korea.

"When you get this many from this many various sizes, it's definitely an infestation," said Department of Agriculture director Joseph Torres.

Agriculture officials said they did not know what species of spider was on the ship. But they said it was a type that was not normally found on Guam and there was concern that it could damage the island's environment.

"It's not on Guam," Mr Torres said. "We don't want it here."

The ship was carrying housing units and accessories which were to be used at a workforce village expected to house up to 18,000 temporary workers.

Before the ship initially docked, officers with the Guam Customs and Quarantine Agency boarded the ship and gave clearance for cargo to be unlocked for offloading, said Bernadette Meno, an administrator for the Port Authority of Guam.

But when port workers saw the spiders, the containers were ordered back on the ship and it was anchored in the harbour. The decision was later made not to let the ship return.

Marianas Steamship Agencies arranged for the ship's arrival and departure. Its vice president, Richard P Sablan, said he would follow the orders of customs, agriculture and US Coast Guard officials.