The al-Tawhid and al-Jihad group, reportedly headed by Abu Musab al-Zarqawi, said in a statement posted on the internet it had killed the second American, believed to be Jack Hensley. It gave no further details.
The message posted on an Islamist website by a contributor who has in the past posted messages in the name of al-Tawhid and al-Jihad, said the group would soon post pictures of the "slaying".
"The sons of our nation have slit the throat of the second American hostage after the deadline passed [on Tuesday] and we will provide you with pictures soon," said the contributor, who goes by the pseudonym Abu Maissarah al-Iraqi.
Hensley's reported death comes a day after the group beheaded the American Eugene Armstrong who was seized along with Hensley and the British Kenneth Bigley last week.
Bigley's brother spoke to Aljazeera.net immediately after Hensley's reported "execution", condemning UK Prime Minister Tony Blair for his policy in Iraq and urging mercy for his brother.
"I utterly condemn the UK stance on foreign policy. Blair is a bully," Paul Bigley said. "The second captive has been executed. Ken is next. Ken is a good guy.
"If Ken is executed, I will still campaign against Blair and his foreign policy," he said.
A British Foreign Office spokesman told Aljazeera.net the government was doing its best to secure Kenneth Bigley's release.
"We are doing all we can ... we are working with the US presence in Iraq," the spokesman said.
An 18 September video grab shows
a man believed to be Jack Hensley
"We have set up a hotline and put out a TV appeal."
The spokesman declined to comment on whether the British government had made efforts to contact the captors.
The Foreign Office travel advice section does not warn against all travel to Iraq which it does for Burundi, Somalia and parts of countries such as Yemen and Sudan, but advises against all but essential travel to Iraq.
Earlier on Tuesday, a CIA official said the agency believed that al-Qaida ally al-Zarqawi was the speaker on the videotape showing the beheading of American captive Armstrong.
"There is high confidence that the voice is indeed of Abu Musab al-Zarqawi," the official said after the CIA conducted a technical analysis of the tape.
The three captives went to Iraq with a UAE-based building company GSCSGulf which has contracts to work at US army bases.
Foreigners working for firms doing business with the US-led force and its appointed interim government have been threatened with death if they continue.