The former Sinn Féin spokesman, Mr Danny Morrison, expressed disbelief at the allegations concerning the British agent code-named Stakeknife.
Reflecting widespread republican shock at the disclosure, he said: "Unless proven otherwise, I'm sceptical of allegations which have as their source British Intelligence."
Mr Gerry Kelly of Sinn Féin said the claims being made about the alleged activites of a British agent working for the Force Research Unit were "extremely serious".
"They add further weight to what is now a compelling argument that the British State operated a policy of assassination against citizens living in the six counties."
He said Sinn Féin would "continue to demand that the British government come clean on their collusion policy".
The UUP leader, Mr David Trimble, said no one should be surprised, and defended the practice of "running" spies within the ranks of paramilitary organisations.
"We all know that the authorities try to penetrate the paramilitary organisations.
"It is the key way in which the paramilitaries have been ground down and brought close to defeat in Northern Ireland.
"The authorities' job is to get intelligence, to get information, and that means turning \ people who are members of paramilitary organisations or finding someone who will penetrate them."
The Northern Ireland Office was also tight-lipped. A representative said it was not normal to comment on security-related issues.
In Dublin, the Green Party called for the extradition to the Republic of the informer.
The leader of the party, Mr Trevor Sargent, said he was "horrified at the reports of the British agent in the IRA carrying out grotesque acts of cruelty including murder in this State, as well as in Northern Ireland." He also called for clarification from the British Prime Minister of the situation.