The BBC's Tom Coulter: "Thorny problem of decommissioning must be overcome"

Emerging for a meeting with UK Prime Minister Tony Blair, he said: "No-one likes the word crisis, but this is a serious situation."

Gerry Adams: "Agreement stymied by unionists"

"Clearly the creditability of the agreement comes more and more into question as the drift continues.

"People are concerned that the promise of the Good Friday Agreement has been stymied by unreconstructed unionists."

Earlier, Northern Ireland's deputy first minister warned the agreement could fall apart within days unless decisions on the vital issues are taken.

Seamus Mallon: "Grave danger that the political process will lose credibility"

Seamus Mallon said he was "very afraid of inertia" and warned details of the shape of new Northern Ireland institutions must be agreed by the beginning of next week.

Seamus Mallon: "Very afraid of inertia"

"If not, we will be in great danger of this process losing credibility," said Mr Mallon, also deputy leader of the nationalist SDLP.

But Mr Adams insisted there was no reason why the institutions could not be sorted immediately.

He said he was pleased the prime minister had come to Belfast to do what he could.

Prime Minister Tony Blair: "Important to get extra momentum behind this process"

Mr Blair is meeting all the main parties during talks on Wednesday to try to narrow the gap between nationalists and unionists over the shape of the new shadow executive for the Northern Ireland Assembly and cross-border bodies.

The sooner all parts of the peace agreement were implemented in full, the better, the prime minister said.

"We have come a very long way and achieved an enormous amount but we have got to go the extra way," he said.

David Trimble: "Some issues remain unresolved"

"The sooner we get the cross-border bodies sorted out the better. And we can do it, we just need people to find a way through it. We have just got to go forward recognising this is the will of the people."

According to the Assembly First Minister and Ulster Unionist leader David Trimble, all sides are understood to be close to agreeing arrangements for north-south institutions.

But there are still differences over the number of government departments, which should be set up in advance of next February's transfer of legislative powers from London to Belfast.

David Trimble: "It is not helpful to generate a sense of crisis when none exists"

The SDLP and most of the other parties are pressing for 10, but unionist are insisting seven is enough.

Arriving at Stormont to meet the prime minister, Mr Trimble said: "There has been a significant amount of progress but some issues remain unresolved."

He said he hoped that the hurdle of weapons decommissioning would also be overcome.

"Society has invested too much in this to allow it to be destroyed by a handful of unreconstructed militarists in the republican movement," Mr Trimble said.

The BBC's Andrew Bomford: Decommissioning of weapons is still the biggest obstacle

A Downing Street spokesman said it was unlikely Mr Blair would be announcing any breakthrough later on.

"We're not going to pretend things aren't difficult," the spokesman said.

"Equally, he wouldn't be there if he didn't believe there was good will and determination to make progress."