The UK and Irish prime ministers have both restated the 24 November deadline is the last chance for politicians to restore Northern Ireland devolution.
Bertie Ahern and Tony Blair want to "quicken the pace"
Tony Blair and Bertie Ahern said failure to meet that deadline would put the assembly in cold storage.
Mr Blair said: "This is the last chance for this generation to make this process work."
They have published a workplan for the politicians in the run-up to that date and are due to return to NI in October.
The premiers met delegations from the DUP, Sinn Fein, SDLP, Ulster Unionists and Alliance at Stormont on Thursday.
"We are convinced that November is the outer limit of an acceptable timeframe," Mr Blair and Mr Ahern said in a joint statement.
"Failure to meet that deadline would be a failure which will put the assembly in cold storage from 24 November.
"That would be very regrettable, but everyone accepts that an assembly subsidised by the public which is not serving its community through active government is simply not sustainable."
Mr Blair said there was "no pressure of an arbitrary sort" they could exert on the political parties.
"We have come a very long way but we need to get the rest of the way now," he added.
Mr Ahern said he wanted to see the institutions up and running as soon as possible.
"The reality of this is that if we don't do this by 24 November then we lose a huge opportunity," he said.
The premiers will return to Northern Ireland following the publication of a report on paramilitary activity.
DUP deputy leader Peter Robinson said they had impressed on Mr Blair and Mr Ahern the need for the assembly to meet.
"If negotiations are going to begin in the autumn, then the assembly is going to have to meet between now and then," he said.
Peter Robinson said the assembly must meet before autumn
"We can not move on to the next stage unless we move in the sequence of scoping the issues, debating the issues and then negotiating the issues."
Sinn Fein president Gerry Adams said he hoped the process could move on.
"The two governments sought to reassure us of their total commitment to the November 24 deadline and their commitment to making this process work," he said.
"We now want them to match that verbal commitment with action in the time ahead."
Ulster Unionist leader Sir Reg Empey said those who had no intention of meeting the 24 November deadline should "have the guts to say so".
"We want to see devolution restored if the conditions are right at the time and we will spare no effort to see that is achieved," he said.
Alliance leader David Ford said his party shared the frustration of the people of Northern Ireland.
"Alliance is prepared to work its socks off until November to get an assembly restored but we need the governments to play their part as well," he said.
Gerry Adams said the PMs "had to bring about change"
Ahead of his meeting, SDLP leader Mark Durkan said: "The taoiseach and the prime minister shouldn't have to be here today trying to make sense out of a lot of the nonsense that is coming from some of the parties.
"I hope the two premiers are clear in their message today that they want to see the parties here get on with it, because that is what the public want to see."
On 15 May, Northern Ireland's politicians took their seats in the Stormont assembly for the first time since its suspension in October 2002.
It has only met a few times since May after its failure to elect a first and deputy first minister.
A cross-party Preparation for Government Committee has been formed, but progress has been slow.
Talks are expected to continue until August, with intensive efforts resuming in the autumn.
If attempts to restore the assembly fail, the two leaders say that in December there will be a "prime ministerial summit to launch new British-Irish partnership arrangements".
The government confirmed the next assembly elections would be postponed until May 2008 if the executive is restored by this date.
Devolved government was suspended over allegations of a republican spy ring. The court case that followed collapsed.
Direct rule from London was restored in October 2002 and has been in place since.
Meanwhile, Mr Blair and Mr Ahern met a cross-community delegation of about 30 children from nine schools in Ballymena.
The meeting was in response to the murder of 15-year-old Michael McIlveen in the County Antrim town last month.
Mr Blair told them they were setting an example which political leaders should follow.