MOGADISHU - 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.

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.

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.