MOGADISHU, Somalia (AP) _ Turkish guards repulsed an attack by Somali gunmen, and U.N. forces today closed an airfield they said used by supporters of a renegarde warlord to ferry in weapons, money and drugs.

Thousands of leaflets dropped by U.N. planes said the K50 airfield 30 miles west of Mogadishu would be shut down indefinitely. The action would stop supplies from reaching fighters for warlord Mohamed Farrah Aidid.

Aidid's Somali National Alliance is blamed for the killings of four American soldiers Sunday and other recent attacks on peacekeepers.

U.S. troops were scheduled to hold a private memorial service today for the soldiers, who died when a remote-controlled bomb ripped apart their Humvee.

Overnight, four Somali gunmen were driven off by Turkish guards as they tried to penetrate perimeter defenses at U.N. headquarters in Mogadishu, said U.N. military spokesman Capt. Jonathan Dahms.

The gunmen retreated under Turkish fire, evidently without injury.

In a second attack after midnight, gunmen sprayed Sword base, a U.S.-manned supply outpost in southwest Mogadishu, with small-arms fire but did not inflict casualties.

Flares were fired as troops reported furtive movements in unlit steets and buildings around U.N. facilities. ''There was a lot of suspicious activity out there,'' Dahms said.

There are 4,000 U.S. troops among some 24,000 peacekeepers in Somalia. The latest contingent of about 1,000 Bangladeshi troops arrived Monday, bringing to 28 the nations in the U.N. coalition.

U.S. military spokesman Maj. Dave Stockwell said the newcomers will reinforce some 13,000 U.N. troops based in this embattled seaside capital.

The closure of the K50 airfield will halt flights from neighboring Kenya carrying khat, a narcotic plant chewed as a stimulant by many Somalis. Khat sellers in a market near U.N. headquarters said they expected an angry reaction from their customers until an alternative supply route was secured.

Weapons are believed to have been sent to pro-Aidid militias from Muslim fundamentalist supporters in Sudan and Iraq.

The U.N. has ordered Aidid's arrest and put a $25,000 reward on his head.

The deaths of the Americans touched off calls for intensified efforts to capture Aidid, who is believed to be in southern Mogadishu, his stronghold.