UN officials said on Friday the operation in the Port-au-Prince slum of Cite Soleil could last days but refused to say when they might enter the heart of the vast shantytown.
Cite Soleil is considered a hotbed of gang violence between loyalists and opponents of ousted President Jean-Bertrand Aristide.
One man was killed on Thursday and at least six others were wounded in gang shootings.
UN military spokesman Lieutenant Colonel Elouafi Boulbars said Jordanian peacekeepers set up 21 checkpoints at entrances to the slum and were searching cars and pedestrians for illegal guns.
"We now control the perimeter of the area," Boulbars said.
Nearby the slum's seaport where Jordanian troops maintain a small base, two young men armed with AK-47s and drinking beer roamed in plain sight as residents crowded around them.
Another gang member with a revolver stopped a carload of journalists before allowing them to enter, indicating gangs still control much of the area.
Port-au-Prince has faced attacks
and looting in recent years
The operation, the first major offensive by UN forces in the capital, comes amid a surge of violence that has killed hundreds since September, including two UN peacekeepers.
Officials fear escalating violence could undermine public trust in general elections in October and November.
In March, UN troops fought bands of armed ex-soldiers in two rural towns that left two peacekeepers dead - the first peacekeepers killed in clashes during their 10-month-old mission. Two former soldiers also died.
Experts say disarming the gangs in the winding streets of Cite Soleil will be far more difficult for UN peacekeepers than dealing with the former soldiers.
"It's like going into an urban jungle," UN spokesman Damian Onses-Cardona said. "You cannot enter Cite Soleil with tanks. It requires a more targeted action."