Physicians for Social Responsibility, an advocacy and policy group that focuses on stopping nuclear proliferation and protecting the environment, said that as many as 52,000 people could die in a nuclear bomb set off on a ship in the port of new York.
Another 238,000 people would be exposed to direct radiation.
The group analyzed three possible scenarios, the nuclear bomb explosion in the New York City port, a dirty bomb explosion in Washington and an attack on a nuclear power plant in Chicago.
In the Chicago scenario, an attack on a nuclear plant, 7.5 million people could be exposed to radiation and 20,000 could receive a lethal dose.
Past official government figures have said a dirty bomb - which is a regular bomb laced with nuclear material - in downtown Washington would kill 180 people and contaminate 20,000 others. But the group gave no estimated figures of their own for Washington.
The report found that the federal government does not have a plan to respond to the surge in medical needs after an attack, has no system to determine whether residents should evacuate or take shelter, has no central coordinating authority to direct response and rescue efforts, and has no plan for sending an adequate number of doctors and nurses to the affected area.
Physicians for Social Responsibility became concerned about what the government would do to help citizens after a nuclear attack when it witnessed the slow response to Hurricane Katrina last year, Dr. Ira Helfand, a member of the organization's board, said at a press conference to release the report.
The results of a nuclear terrorist attack 'would make Katrina look like a rainstorm,' Helfand said.
But the Department of Homeland Security, charged with protecting the country against terrorist threats, dismissed the report, saying it 'fails to grasp reality.'
The department is focused on preventing such an attack by monitoring US ports and borders, Russ Knocke, a department spokesman, told the Deutsche Presse-Agentur, dpa in an interview.
'It's far better to worry about preventing that kind of attack and that's where we're focused,' Knocke said.
Though nothing could be done to save those killed immediately, a coordinated response to treat others exposed to fallout from the bomb could save thousands of lives, the report says.
'The governments' ability to quickly and effectively evacuate communities or shelter populations downwind will be the single most important factor in minimizing the casualties and injuries,' the report says.
The lack of a plan to tell residents whether to evacuate or stay in their homes is especially dangerous, the group says. Most people's instincts would be to flee the area, but most residents will be safer from nuclear fallout if they seek shelter in the event of an attack.
The group plans to deliver copies of its report to members of Congress and the Department of Homeland Security, with the hope of striking a chord there, Will Callaway, senior policy coordinator for the group, told Deutsche Presse-Agentur, dpa.
Knocke, the department's spokesman, says it is not true that there has been no focus on preparation.
He cited the department's 'Ready Campaign,' which aims to get Americans thinking about what they would do in case of a terrorist attack.
The government has also stepped up the screening of shipping containers coming into the United States, and now scans 70 per cent of the containers for radioactive material, Knocke said. The department aims to increase the screening to 98 per cent by late 2007.
The report concludes that the government still has much to do in order to deal with the fallout of a nuclear terrorist attack. The US government needs to designate a central coordinating authority to direct the response and prepare for an attack; store medical supplies, radiation protection equipment and monitoring devices in high-risk areas; and establish a plan to communicate with the public and training medical first responders, the report concludes.