Barack Obama, the US president, has mobilised the military to help deal with the spreading oil slick in the Gulf of Mexico.

Endangered birds and animals in four US states are threatened by the spill that Obama said oil giant BP is "ultimately responsible" for.

Experts fear the pollution could be worse than the Exxon Valdeez spill more than 20 years ago.

The first traces of oil from the slick have now reached the mouth of the Mississippi river, but out to sea, the situation is much worse.

Planes spraying dispersant chemicals on the oil slick are making little progress - while BP's crews fail to cap the ruptured well-pipe on the sea bottom and called for help from US Navy submarines.

Al Jazeera's Sebastian Walker reports from Louisiana, where shrimp fishermen fear the worst.