The United Kingdom will "fundamentally change" its relationship with the European Union in 2016, after talks with EU leaders in Brussels, David Cameron says.
The UK prime minister said he was a "step closer" to getting the reforms he wanted, which he will put to the British people in a referendum.
He said it would be "pretty tight" to get a deal by February as he had hoped - but it would happen next year.
Eurosceptics said his reform demands were trivial.
And Labour leader Jeremy Corbyn said he believed little of substance would be secured in the next six weeks before EU leaders meet again.
Mr Cameron is demanding change on four issues, including stopping in-work benefits for EU migrants in the UK for four years.
German Chancellor Angela Merkel said leaders "all want a compromise".
Mr Cameron wants to get a new deal before putting the question of the UK's membership of the EU to an in-out referendum of British people by the end of 2017.
As well as controls on migrant benefits, he is also demanding protection for countries outside the eurozone on financial matters, measures to boost competitiveness and an exemption for the UK from the drive towards an "ever-closer union".
BBC political correspondent Alex Forsyth said she understood a number of ideas were "kicked around the dinner table" by EU leaders at the summit on Thursday night.
There was suggestion among them that a legally binding agreement - and potentially treaty change down the line - would be possible, she said.
However, officials only had a very short timeframe in which to hammer out the technical details by February, she added.
Late on Thursday evening, Mr Cameron emerged from the talks and told reporters a "big step forward" had been taken for a "better deal" for Britain.
"There's still a lot of hard work to be done but there is a path through this," he said.
The prime minister added: "In terms of welfare, no, I haven't put any other proposals on the table - I have put my four-year proposal on the table and it remains on the table."
"Nothing is certain in life or in Brussels but there is a pathway to a deal in February," he said.
Treaty change 'possible'
European Council president Donald Tusk said Mr Cameron had set out his position, particularly on benefits and free movement.
Leaders had voiced concern but were willing to look for compromises, he said.
"Building on this positive debate we agreed to work together to find solutions in all four baskets raised by Prime Minister Cameron," he said.
German Chancellor Angela Merkel said: "I am optimistic because we all want a compromise. But work on substance needs to be done. Treaty change might be possible. Not now, but perhaps later."
French President Francois Hollande said there could be adjustments over Mr Cameron's demands, but EU rules and principles must be respected.
European Commission president Jean-Claude Juncker warned that all four of Mr Cameron's demands were difficult, and intensive negotiations were needed before February.
He said: "I'd like to warn you of the illusory impression that there are three easy questions and one tricky one.
"There are four tricky questions, each one covers further questions and we have to consider all of those until February."
BBC political editor Laura Kuenssberg said Mr Cameron's appeal to his EU counterparts was a wider political call for help - he referred to the tightening polls at home, and the support of those who want to stay in the EU appearing to erode.
"To pass the political test he set himself, he needs his fellow EU leaders to do more than show willing but to accept at least strands of his ideas", she said.
Thursday's talks over dinner were the first time EU leaders had discussed Mr Cameron's reform proposals in detail.
France and Germany are among a number of countries that say any deal must safeguard the free movement of people.
Also on the agenda at the European Council meeting were the migrant crisis, climate change and the fight against terrorism.