Two suspicious containers from Pakistan forced evacuation of a Seattle port terminal in North America after a bomb-sniffing dog indicated they could contain explosives.

However no explosives, bombs, chemical or biological agent was found, officials said.

The suspicion was raised on Wednesday, when scanning by a gamma ray device suggested presence of articles which were not declared in the inventory.

A bomb-sniffing dog, too, reacted and led the officials to order evacuation of the terminal.

Officials said the containers were supposed to contain oily rags, which are often shipped internationally for recycling or use in packaging.

The gamma ray device determines the density of the cargo and gives clue to whether it matches with the manifest. No explanation was available as to why the dog reacted the way it did.

A bomb squad was summoned, which detonated small charges to create tiny holes for the investigators to determine what was inside. They found no radioactive material or explosives but it was not clear whether further search would be carried out.

However, an official held out the possibility of explosives used to make holes in the containers might have detonated small explosives inside.

The two were among 70 containers from the vessel set aside for closer inspection based on several risk factors, including country of origin, merchandise and weight.

The vessels that brought the suspicious containers is owned by China Shipping, officials said. It originated in Hong Kong and reached Seattle after making stops in China and South Korea.

The Pakistani containers were believed to have been transferred from a feeder vessel somewhere in Asia.

As the alert was raised, security officials set up a 600-meter perimeter around the terminal and the coast guard a 270 meter perimeter in water.

About two billion tons of freight arrives in the United States each year but only a fraction of about 5 percent is examined. Recently, there have been calls by the lawmakers to tighten security and examine all containers entering the country.