In this story:

• 'We're operating as islands'
• Hope of finding survivors fading
• Related sites and stories

ARMENIA, Colombia (CNN) -- As mobs of desperate survivors of a devastating earthquake looted stores in smashed cities and towns in western Colombia, thousands of troops poured into the area to stop them.

Colombian President Andres Pastrana arrived Thursday in Armenia, the city hardest hit by Monday's magnitude 6 quake, to oversee relief and rescue efforts.

"I will not rest until I see that our emergency plan is working efficiently," Pastrana said in a nationally televised address.

Pastrana said 290 tons of aid had been sent to the region, which needs 150 tons of food daily. The military said it was handing out 120,000 military rations to the hungry.

But confusion and a shortage of trucks and workers stymied distribution efforts, leaving tons of supplies, from potatoes to shampoo, piled in warehouses.

"We don't have 600 or 700 state officials to bring it out to the communities," said Fernando Medellin, national director of Pastrana's aid organization, the Solidarity Network.

While a plan existed to distribute aid regionally, the only way for most neighborhoods and towns to get help was for residents or officials to come and get it, Medellin acknowledged.

Local officials arranged direct air shipments of drinking water, canned food and milk to a few stricken towns. Hard-hit Calarca, just south of Armenia, got a shipment only after its mayor showed up at the airport, pleading for help.

"We're operating as islands. A lot of effort is going to be wasted," said aid worker Ricardo Trujillo.

In Armenia, where gangs of looters continued to steal food and clothing, Pastrana imposed a dusk-to-dawn curfew. About 4,000 soldiers and police were sent in to patrol the city.

"I will not permit criminals from other areas to impose chaos," Pastrana said. "There are still a few people trying to sow violence, disorder and confusion."

Despite the police presence, Armenia's main supermarket was set ablaze Thursday. Soldiers fired tear gas at looters as they emerged from a food warehouse. Other troops shot automatic gunfire into the air.

At least 881 people had died and more than 3,500 were injured in the quake, the Red Cross reported. Some 200,000 were left homeless.

"The magnitude of this tragedy exceeds all initial calculations," Pastrana said.

Meanwhile, a strong aftershock from the quake rocked Armenia early on Friday, but no injuries or serious damage were reported. But it triggered widespread panic among jittery survivors. The tremor was one of at least 37 powerful aftershocks from Monday's quake.

Search and rescue teams from the United States, Mexico, Japan, Russia, Chile, Spain, Britain, Germany and France scrambled over and into collapsed buildings, looking for more victims.

Dozens, if not hundreds, were still missing, but emergency crews said chances of finding any more survivors were limited.

"A few may still be pulled out, but most of what we're going to see over the next few days is frustration," said Paul Bell, who heads a 65-member rescue team from Dade County, Florida, which helped in rescue efforts after the Oklahoma City bombing and the 1988 earthquake in the former Soviet republic of Armenia.

Many grieving survivors surveyed their damaged homes, hoping to salvage whatever they could.

Brasilia resident Francisco Gutierrez, trying to pull a faucet off a chunk of his bathroom wall, said he lost everything in the quake.

"Only God knows how I got out of there alive," he said. His wife was crushed by a falling wall.

He took one last look at his shattered house before leaving to live with his daughter, whose home across town was not damaged.

David Frank Rodgers, a New Yorker born of Colombian-American parents, flew to the region to help his in-laws leave.

"It's pretty sad," he said. "Not because of the house, but because of the people. They lost a lot of friends. Almost everybody died.... Everybody is dead."

Correspondent Harris Whitbeck, The Associated Press and Reuters contributed to this report.