BELFAST (Reuters) - Britain on Tuesday called new peace proposals on Northern Ireland a breakthrough but said tough negotiations lay ahead to secure the agreement of all political parties on the details. The blueprint unveiled by the British and Irish governments on Monday injected new life into painfully slow talks aimed at ending decades of conflict over British rule of Northern Ireland. Given a cautious welcome by most of the parties attending the talks, it envisages an elected Northern Ireland assembly, an all-Ireland ministerial council and sees Dublin dropping its territorial claim to the province. Politicians from eight pro-British Unionist and Republican parties, who want a united Ireland, were due to get down to detailed discussions on the document on Tuesday after a Christmas recess that had been marred by an rash of guerrilla violence which killed four people. Speaking in Japan, from where he played a direct part in drawing up the plan, British Prime Minister Tony Blair said it marked a move from "symbolism to substance." U.S. President Bill Clinton welcomed the initiative and urged Northern Ireland's political parties "to seize the moment and begin negotiating details of an agreement." British newspapers greeted the initiative with enthusiasm. The Independent in an editorial called the ideas "radical, daring and even visionary". But the blueprint was seen as marginalising the IRA's political wing Sinn Fein, which entered the talks for the first time in September. Sinn Fein, which rejects any British role in the future government of Northern Ireland, offered the coolest response saying only that it would give the proposals careful study. Mowlam insisted that the blueprint was open to negotiation and was not an attempt to dictate the terms of any settlement. "There will be bits in the paper that Sinn Fein dislikes, there will be bits that Unionists and Loyalists dislike," she said.