MEXICO CITY - Mexico, the biggest buyer of US chicken, has halted imports of live birds and eggs from the US state of Iowa due to an outbreak of deadly bird flu there, the Mexican government said on Tuesday.

Mexico and other major countries last month imposed new export restrictions on poultry products from various US states, but a new outbreak has hit top US egg-producing state Iowa.

Iowa found a lethal strain of bird flu in millions of hens at an egg-laying facility on Monday, the worst case so far in a national outbreak that prompted Wisconsin to declare a state of emergency.

Bird flu, also called avian influenza or AI, is a viral disease that infects birds. Officials believe wild birds are spreading the virus but they do not know how it is entering barns.

Shares of several leading meat companies' in the United States fell on Tuesday on concerns over the Iowa outbreak.

Mexico's agriculture ministry said it and animal health body SENASICA had tightened controls and monitoring of migratory wild birds, and it said they were in constant contact with bird farms in Mexico to detect any suspicious cases domestically.

The ministry emphasized that Mexico is self sufficient in the production of eggs for consumption, and that Mexican producers import fertilized eggs used for breeding.

Iowa was already among 12 states that have detected bird flu in poultry since the beginning of the year. The other states are Arkansas, Idaho, Kansas, Minnesota, Missouri, Montana, North Dakota, Oregon, South Dakota, Washington and Wisconsin.