Downing Street has refused to confirm reports that the IRA is secretly re-arming and advising other terrorist groups.

The row broke on Monday ahead of a debate in the Northern Ireland assembly in which first minister David Trimble demanded that Sinn Fein explain more about the IRA's links to Marxist guerrillas in Colombia.

Trimble said that Northern Ireland secretary, Dr John Reid, should take action against the party for breaches of the IRA ceasefire.

He said Dr Reid should consider putting Sinn Fein on notice in the same way he did with the loyalist Ulster Defence Association.

The first minister said that Sinn Fein had to give answers on IRA involvement with IRA terrorists, an increase in terrorist activity in Northern Ireland and the break-in at Castleraegh police station.

"We have to acknowledge...that there have been serious breaches of the IRA ceasefire," said Trimble.

The leader of the Democratic Unionists, the Rev Ian Paisley, welcomed "the fact that the leader of the Ulster Unionist Party had woken to the fact that the IRA is still engaged in murdering".

He accused the IRA of re-arming with high-tech weapons bought from Russia and called on the assembly to take action against the republicans as a result.

"Let me say to this house that the time has come when we have to realise that the secretary of state has no power, and that this is the house where the power lies," he said.

"The time has come when the people of this government should remove them [republicans] from this government and remove them from this house."

Speaking ahead of the debate, Sinn Fein MP and assembly member Martin McGuinness said the IRA had made a commitment to peace.

"In the course of the last five years, the IRA hasn't even fired a shot at a member of the RUC. They haven't even fired a shot at any member of the British Army...That's the commitment that has come from the IRA."

McGuinness, who is the assembly's education minister, blamed faceless "securocrats" - members of the British security services - for attempting to wreck the peace process and attacked David Trimble's remarks.

"I think there's every danger of us reaching a huge crisis within this process when I hear for example Mr David Trimble talking about the nuclear option. We know what happened at Hiroshima, we know what happened at Nagasaki. Why are we talking about making the North of Ireland effectively a political wasteland?" he said.

Number 10 said information from the security services, including the Police Service of Northern Ireland, indicated that the ceasefire was still being observed by the IRA.

The prime minister's official spokesman argued that the second act of arms decommissioning showed the IRA leadership was committed to the peace process.

Irish premier Bertie Ahern fuelled the row last week when he claimed that Sinn Fein had yet to give up its violent past south of the border.