A second strong earthquake hit Haiti today, waking people from their beds and sending others running into the streets in panic.
The US Geological Survey said the quake measured 6.1, and hit a country already ravaged by an earthquake eight days ago.
It came amid rising frustration from one aid agency about the delay in getting lifesaving supplies into Haiti, with doctors saying they are having to buy their own saws to carry out amputations. Another agency estimates 20,000 people are dying every day who could be saved by surgery.
The quake was centred 26 miles north-west of Jacmel, and the US Geological Survey said it struck at a depth of 13.7 miles.
Ed Pilkington, the Guardian correspondent in Port-au-Prince, 35 miles from the epicentre, said this morning's quake was strong enough to be felt in the Haitian capital.
Pilkington tweeted: "just been woken by my bed shaking in #Haiti. another after shock."
The tremor caused panic and the Associated Press said "wails of terror" could be heard from people as they fled buildings already left in ruins by last week's quake. It was not immediately possible to assess what additional damage the new quake may have caused.
The UK's Disasters Emergency Committee said: "The news of today's tremors are of grave concern. We are assessing the impact of the aftershock with our field staff to see if there has been further large-scale disruption to survivors and aid operations."
Aid worker Tara Livesay said the aftershock lasted for 15 seconds: "All of a sudden the house was rocking. It was the strongest aftershock we've felt yet. I cannot begin to describe how totally afraid everyone here is ... these aftershocks stir everything back up to the surface."
Last week's huge quake killed an estimated 200,000 people in Haiti, left 250,000 injured and made 1.5 million people homeless. A big international aid effort has been launched, but is struggling with overwhelming logistical problems.
A statement from the aid group Partners in Health, co-founded by the deputy UN envoy to Haiti, Dr Paul Farmer, said the group's medical director estimated 20,000 people were dying each day who could have been saved by operations.
One leading aid agency went public with its frustration at the organisation of the relief operation. Médecins sans Frontières said a cargo plane carrying 12 tonnes of vital supplies had been turned away from landing in Port-au-Prince three times since Sunday.
"We have had five patients in Martissant health centre die for lack of the medical supplies that this plane was carrying," said Loris de Filippi, the emergency co-ordinator for the MSF's Choscal hospital in Cité Soleil.
"I have never seen anything like this. Any time I leave the operating theatre I see lots of people desperately asking to be taken for surgery. Today, there are 12 people who need lifesaving amputations at Choscal hospital. We were forced to buy a saw in the market to continue amputations. We are running against time here."
"It is like working in a war situation," said Rosa Crestani, the MSF medical co-ordinator for Choscal hospital.
"We don't have any more morphine to manage pain for our patients. We cannot accept that planes carrying lifesaving medical supplies and equipment continue to be turned away while our patients die. Priority must be given to medical supplies entering the country."
Haiti's president, René Préval, said the biggest problem facing his people was getting the aid that had arrived from abroad to where it was needed.
Préval told Radio France International: "When it arrives, the question is: where are the trucks to transport it, where are the depots?"
Sophia Eltime, a 29-year-old mother of two who has been living under a bedsheet with seven members of her extended family, begged for help: "We need so much. Food, clothes, we need everything. I don't know whose responsibility it is, but they need to give us something soon."
Search-and-rescue teams have emerged from the ruins with some improbable success stories including the rescue of 69-year-old woman who said she prayed constantly during her week under the rubble.
Ena Zizi had been at a church meeting at the residence of Haiti's Roman Catholic archbishop when last week's earthquake struck, trapping her. She was rescued yesterday by a Mexican disaster team.
Zizi said that after the quake, she spoke back and forth with a vicar who also was trapped. But after a few days, he fell silent, and she spent the rest of the time praying and waiting.
"I talked only to my boss, God," she said. "And I didn't need any more humans."
Authorities said close to 100 people had been pulled alive from wrecked buildings by international search-and-rescue teams. Efforts continued, with dozens of teams sifting through Port-au-Prince's crumbled homes and buildings for signs of life. But survivors expressed frustration with the slow pace of the relief effort.
The World Food Programme said more than 250,000 ready-to-eat food rations had been distributed in Haiti by yesterday, reaching only a fraction of the 3 million people thought to be in desperate need.
The WFP said it needs to deliver 100m ready-to-eat rations in the next 30 days. Based on pledges from the US, Italy and Denmark, it has 16m in the pipeline.
So far, international relief efforts have been disorganised, disjointed and insufficient to satisfy the great need. Governments have pledged nearly $1bn (£640m) in aid, and thousands of tonnes of food and medical supplies have been shipped. But much remains in warehouses, or diverted to the neighbouring Dominican Republic. The damaged port and impassable roads complicate efforts to get aid to the people.
The US defence secretary, Robert Gates, said the military would send a port-clearing ship with cranes aboard to Port-au-Prince. It will be used to remove debris that is preventing many larger ships carrying relief supplies from docking.
The UN security council voted unanimously yesterday to approve an additional force of 3,500 police and soldiers for Haiti, amid rising concern over outbreaks of looting by desperate survivors and the re-emergence of notorious gang leaders who escaped when the country's prisons collapsed.
