MOGADISHU march 09 Reuters) -- Somalia's Islamist al Shabaab rebel group vowed on Monday to defend itself against any government offensive and to subject the U.S. to a failure worse than one it suffered in 1992 should it back the attack.

Unconfirmed media reports said over the weekend American military experts were advising Somali government troops ahead of their push to oust the rebels from the capital.

"We shall defend ourselves if they (government forces) attack us," al Shabaab spokesman Sheikh Ali Mohamud Rage told a news conference.

"They typically repeat offensive words, why don't they attack us? We shall never enter talks with the so-called government. America can do nothing to us -- it will face something worse than its failure in 1992 in Somalia."

American troops who were part of a U.N. humanitarian mission to Somalia in 1992 and 1993 were forced to pull out after Somali militia killed several marines in an attack on a U.S. helicopter.

The U.N.-backed administration of President Sheikh Sharif Ahmed is keen to drive the rebels out of the capital as a key part of restoring stability in the Horn of Africa nation which has had no effective central government for 19 years.

Sources said his troops are likely to attack rebel positions once the president returns from a trip to Europe and signs a political deal with the Ahlu Sunna Waljamaca, a moderate group which backs the government.

Al Shabaab will likely count on the help of another rebel group, the much smaller Hizbul Islam, which is led by Sheikh Dahir Aweys.

Al Shabaab, which Washington views as al Qaeda's proxy in the region, seeks to impose a strict version of Muslim law in the country.

Last month, it asked the U.N. food agency, World Food Programme, which has been central to the international community's response to the country's massive humanitarian crisis to leave.

"WFP will never operate again in the areas under our control," Rage said.