Amec, Bovis Lend Lease, WSP and Skanska among companies that volunteered to look for survivors.
British firms are at the centre of the rescue and clean-up effort in Manhattan and Washington after the devastating terrorist attacks.
The destruction of the twin towers of the World Trade Centre has left officials with the Herculean task of clearing away the debris. It is estimated the clean-up may take as long as a year.
Amec and Bovis Lend Lease are together controlling the salvage operation around the remains of the south tower.
Amec has also won the contract to rebuild the Pentagon in Washington.
New York's department of design and construction is co-ordinating the overall operation and has divided the WTC site into quadrants. Amec and Bovis Lend Lease are overseeing work in the south-west quadrant (see map, below). American firms are overseeing work in the other areas.
David Paterson, Amec's senior vice-president for corporate communications, said: "We immediately volunteered our services and were on site with cranes and heavy lifting equipment within hours of the tower collapse.
"The key issue is safety, as this is an unprecedented site in terms of scale and challenge." Paterson added: "This is the end of a rescue effort and the beginning of a salvage operation."
The company has 200 professional managers and 1000 tradesmen and labourers working in 12 hour shifts around the clock in New York. Paterson said Amec had plenty of help at the site, but expert personnel could be drafted in from the UK.
Bovis Lend Lease was another firm to offer its services immediately after the towers fell; it has about 60 managers on site. A spokesperson for the company said: "This effort goes on 24 hours a day, seven days a week, with the number of people we have there growing daily.
"We have had two guys there non-stop since Tuesday – they just don't want to go home and are sleeping on site."
Bovis has also built two emergency centres, one for the Red Cross and New York police department and the other as a command centre.
Tall buildings specialist engineer Cantor Seinuk, bought by WSP last year, has six men working around the clock in teams of three on the Manhattan effort.
Cantor Seinuk managing director Ahmad Rahimian said the company's team was mainly working around the south tower, advising contractors, the fire department and the police on how to move through the rubble. The team is also advising on the safest location for cranes to come in and lift the debris
Rahimian added that it was likely that some of the other buildings in the World Trade Centre would have to be pulled down.
Along with fellow director Jeffrey Smilow, Rahimian worked through the night at the site on Thursday.
Cantor Seinuk has begun talks with several tenants and landlords at neighbouring buildings to make structural assessments.
One of the company's engineers was particularly lucky last week: he survived the attack, despite being in a lift at the centre at the time.
Parsons Brinckerhoff, a New York-based engineer with offices in the UK, is also helping with the rescue and recovery operation. It has pledged to assist the rebuilding of the city that its firm has "proudly called home" for 116 years.
Contractor Skanska has been asked by the City of New York and the Port Authority to bring resources into the city. It will help with the clean-up operation once the firefighters have completed their rescue effort.
Buro Happold, Bechtel and Arup are also involved in rescue work.
<B>Rebuilding the Pentagon</b>
Amec was refurbishing one side of the US defence department headquarters when the adjacent "wedge" was struck by a hijacked airliner. It will now work on rebuilding that section.
The company said security considerations prevented it disclosing the precise nature of the rescue effort or the number of workers it had at the site.
We have had two guys there non-stop since Tuesday – they just don’t want to go home and are sleeping on site
Bovis Lend Lease spokesperson
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Industry in front line of rescue and clean-up effort