Long backlogs of patients injured in the Haiti earthquake are building at medical clinics as aid workers warn of more deaths from untreated injuries and disease in overcrowded makeshift camps.

The warnings come as the US prepares to send 4,000 more troops to Haiti to help the relief effort. The new contingent of soldiers and marines diverted from deployments in the Gulf and Africa will take the US presence to about 16,000 troops.

Earth-moving equipment is being used in an effort to speed up the burial of 200,000 people estimated to have died in last week's disaster while estimates of those made homeless have leapt by a third to 2 million.

Governments around the world have so far pledged a total of nearly $1bn (£610m) aid, the Associated Press estimated.

The further US support came after a strong aftershock jolted Port-au-Prince yesterday, sending people fleeing on to the streets and complicating relief efforts.

A magnitude 5.9 quake, the most powerful aftershock since the 12 ­January cataclysm, rattled ruins in the capital and sowed panic but caused no serious reported damage or casualties.

Seismologists said the epicentre was about 35 miles south-west of the city and the focus was six miles deep. They warned of possible stronger aftershocks to come as the earth adjusted to new stresses caused by the original quake.

Greg Elder, deputy operations manager for Médecins sans Frontières in Haiti, said: "The next health risk could include outbreaks of diarrhoea, respiratory tract infections and other diseases among hundreds of thousands of Haitians living in overcrowded camps with poor or non-existent sanitation."

About 80,000 people are thought to have been buried in mass graves so far. Earthmovers are being used to bury 10,000 victims in a day as the grave digging continues on a hillside outside the capital. Casualty figures from the European commission show a significant increase in the number of people believed to be homeless, up from 1.5m yesterday. Hundred of thousands are thought to be injured or in need of urgent aid.

The head of the World Food Programme Josette Sheeran and EU aid chief Karel De Gucht were planning to visit Haiti today to assess for themselves the scale of the catastrophe.

Meanwhile, fears of more quakes remain. "Sometimes [they] die out very quickly. In other cases they can go on for weeks, or if we're really unlucky it could go on for months," Bruce Pressgrave, of the US Geological Survey, told AP.

Yesterday's dawn quake lasted about eight seconds, produced clouds of dust and sent people screaming from shelters. One woman with a heart condition died from fright. Others simply slept through it.

The Haitian government said it would send a response team to Petit-Goâve, near the epicentre, where seven buildings reportedly collapsed. "We know they are going to need some help," said the prime minister, Jean-Max Bellerive.

Combined with drizzle which has compacted debris, the tremor compounded the multiple difficulties of teams seeing survivors and corpses buried amid semi-collapsed structures.

Navy officials said yesterday that the three-ship USS Nassau amphibious ready group left port on Monday for its regular deployment but has been told to go Haiti instead for the earthquake relief effort.

The group is picking up marines in North Carolina and will include 2,000 sailors and 2,000 marines when it gets under way for Haiti.

Last week Hugo Chávez of Venezuela and Daniel Ortega of Nicaragua, both leftist presidents, accused Washington of masking imperialism as aid. Haiti has welcomed the Americans but turned down an offer of 800 troops from the Dominican Republic, according to western diplomats because of historic tension between the neighbours.

Yesterday food, water and medical care seemed marginally more plentiful on Port-au-Prince's streets. "Supplies are beginning to get out to the people," the US defence secretary, Robert Gates, said during a visit to India. Banks are due to reopen shortly and some money transfer agencies are already functioning.

A separate criticism of the international response came from aid agencies who called for an urgent moratorium on all adoptions from Haiti, saying that taking children out of the country at present risked causing long-term damage to already vulnerable children.

Thirteen agencies working together in the UK under the umbrella of the Disasters Emergency Committee said many children apparently orphaned by the quake will have surviving relatives, and that efforts should instead be concentrated on reuniting families. Unicef and SOS Children, the world's largest orphan charity, also cautioned against hasty plans for large-scale adoption.

Miami's Catholic church has declared plans to fly hundreds of children to a new life in the US, while Edward Rendell, the governor of Pennsylvania, flew to Port-au-Prince to evacuate 54 orphans from the city after being told by Haiti's ambassador to the US that his presence could cut through red tape.

A party of Dutch social workers and immigration officials landed in Haiti earlier this week to evacuate 100 children whose adoptions were already under way, while the US has waived visa requirements for Haitian children already going through the process and Canada is also fast-tracking adoptions.

Jasmine Whitbread, Save the Children's chief executive, said: "The vast majority of the children currently on their own still have family members alive who will be desperate to be reunited with them and will be able to care for them with the right support.

Taking children out of the country would permanently separate thousands of children from their families – a separation that would compound the acute trauma they are already suffering and inflict long-term damage on their chances of recovery."

Save the Children has teams in Haiti working on identifying lone children and tracing family members. World Vision's chief executive, Justin Byworth, said a wave of adoptions could leave children vulnerable to trafficking and abuse. Adoptions already in progress should go ahead, argued the agencies, as long as they conformed to local and international law.