NAIROBI, Kenya – Islamists who took control of Somalia’s anarchic capital this week declared war on “infidels” yesterday and warned that any American intervention in Mogadishu would be “disastrous.”

The chairman of the city’s Islamic court union, Sheik Sharif Sheik Ahmed, warned that any action from Washington would fail, echoing the botched 1993 operation that left 18 American servicemen and 300 Somalis dead, and was used as the basis for the movie “Black Hawk Down.”

“If U.S. forces intervene directly against us in Mogadishu, then we are ready to teach them a lesson they will never forget and repeat their defeat in 1993,” he told the Saudi-owned pan-Arabic daily Asharq al-Awsat.

Militia loyal to hard-line Islamic courts yesterday celebrated the capture of Mogadishu after a three-week battle with secular forces said to be backed by Washington.

It is widely believed that the courts’ leaders are at best sympathetic to Al Qaeda and at worst actively support its agents.

On Tuesday, President Bush expressed concern about the fall of most of the city to the Islamist forces and vowed that Somalia would not become a haven for terrorists similar to Afghanistan under the Taliban.

The city’s most senior Muslim cleric, Sheik Nur Barud, called on Somalis to crush secular resistance to Shariah law. “All Somalis must defend the Islamic courts because this is not inter-clan fighting but war with the infidels,” he said in a speech aired by local radio stations.

The defeated side, a loose alliance of businessmen and warlords who were the only opposition to the Islamic courts, retreated yesterday to defensive positions near Jowhar, 55 miles north of Mogadishu. Although badly routed, they vowed not to let the town fall. At least 347 have been killed and 1,500 wounded since violence flared again in February.