Ken Livingstone did not "directly blame" the US for the Boston bombings, Labour insisted after Tory demands for him to be axed over controversial comments about terrorism.
Mr Livingstone sparked controversy by suggesting the brothers who launched the attack on the city's marathon acted "out of anger and demand for revenge" over American foreign policy.
US-born Conservative MP Brooks Newmark wrote to Opposition leader Ed Miliband calling on him to condemn the remarks, made by the former London mayor on a phone-in on Iranian channel Press TV.
He called for the Labour veteran to be removed from the party's National Executive Council (NEC) over the "disgusting" remarks that suggested the attacks were "justified and excusable".
But a Labour spokesman said: "The Labour Party does not believe that US policy is responsible for the Boston bombings and it would be inaccurate to suggest that Ken Livingstone was directly blaming the US for the bombings."
Mr Livingstone said on Friday in comments on the Tehran-funded channel that were criticised in the US media: "Very often, people get incredibly angry about injustices that they see."
Referring to "torture at Guantanamo Bay, at Bagram airbase", rendition, and "ignorance in the Bush White House about Islam" and Middle East issues, he said: "People get angry - they lash out.
"It's the whole squalid intervention that has disfigured the record of the Western democracies.
"I think this fuels the anger of the young men, who - as we saw in Boston - went out, and, out of anger and demand for revenge, claimed lives in the West."
Dzhokhar Tsarnaev, 19, is accused of carrying out the attack - which killed three and injured more than 260 last month - with his brother Tamerlan, who died in a shoot-out with police.
In his letter Mr Newmark said it was "wholly reprehensible for Mr Livingstone to link the Boston bombings to Western intervention in the Middle East.
"In doing so, he suggests that the actions of the two Boston bombers were justified and excusable," he told Mr Miliband.
"I am sure that you will also agree with me that the only two people responsible for the bombings were the terrorists themselves."
Removing Mr Livingstone from the NEC would "make clear that neither you nor the Labour Party condone such beliefs," he added.