The US Coast Guard is asking ships operating in Hilo Harbor in Hawaii to watch out after a container spill Tuesday.

The agency said as many as 12 containers fell off the Young Brothers-owned 4,511-gt container barge Ho Omaka Hou (built 2007).

Two have since been retrieved and another two located with the help of Cates Marine and American Marine, the latter of which is using a 250-ton crane to lift the containers out of the water. The remaining nine are still unaccounted for.

"Plans are in place to begin operation to remove the recovered containers from the water in Hilo Harbor tomorrow morning," the Coast Guard said. "Once begun, offloading in expected to take several days.

The Coast Guard said it broadcast a notice to mariners to use extra caution when transiting the area, which remains in effect. It is advising to keep a sharp lookout for any containers that may present a hazard to navigation.

It said it conducted a flight to try and locate the remaining containers, but was unable to.

Honolulu-based Young Brothers, which provides inter-island freight services in Hawaii, and the Coast Guard are working together to investigate the incident, which has been deemed a reportable marine casualty.

The National Transportation Safety Board has been advised and is said to be acting in a supportive role.

Young Brothers is also conducting an investigation into the incident.

The Coast Guard said none of the overboard containers contained hazardous materials.