David Cameron and President Nicolas Sarkozy have described their horror at events in Syria and pledged support including food rations for the country's citizens.
The summit came as the UN General Assembly overwhelmingly backed a resolution calling for President Bashar al Assad to end the violence which has claimed thousands of lives.
Both Mr Cameron and Mr Sarkozy forged a close working relationship during the Libyan uprising, which began exactly a year ago.
The French president said they would not accept a dictator massacring his people and wanted to help.
Food rations and greater humanitarian assistance is planned.
But he added: "The revolution cannot come from outside, it must be born from within."
His British counterpart described the situation as "horrific" and accused the regime of "butchering" citizens.
Mr Sarkozy began the news event by emphasising the relationship between the two countries remained "excellent".
Both acknowledged they had some differences of opinion.
However, Mr Cameron highlighted their agreement on foreign affairs and the economy and said joint work on the "most sensitive dossiers of all, including nuclear" demonstrated their close co-operation.
They discussed how to help the Free Syrian Army without supplying troops.
Both men are said to be eager to give the resistance movement logistical and diplomatic support, and help human rights groups gather evidence against alleged atrocities.
The Prime Minister is also in Paris to seal a landmark deal between Britain and France to co-operate on nuclear energy, paving the way for the construction of a new generation of power plants in the UK.
Deals worth more than £500m between British and French companies will allow work to start on new facilities, creating more than 1,500 jobs across the country.
The Prime Minister said the agreements were "just the beginning" of investment which the Government believes could be worth £60bn and create 30,000 jobs.
The two leaders held a mini summit, designed to show a more "cordiale" relationship after a series of diplomatic spats.
As well as Syria and nuclear energy, other topics understood to have been on the agenda include Afghanistan, the debt crisis in the Eurozone, and an Anglo-French defence partnership .
The meeting was a delayed annual gathering between the two governments and Mr Cameron is in the country with a clutch of senior ministers.
The British contingent included Deputy Prime Minister Nick Clegg, Defence Secretary Philip Hammond and Energy and Climate Change Secretary Ed Davey.
Rolls-Royce will sign a £400m deal with French energy giant Areva to supply services to the first EPR (next generation) reactor at Hinkley Point in Somerset, with a commitment for future EPR sites in the UK.
Rolls-Royce will build a dedicated factory in Rotherham and the deal will underpin more than 1,200 jobs in the company and its supply chain.
Meanwhile, France's EDF will conclude a £100m agreement with Keir/BAM Nuttall for preliminary works at Hinkley Point - the first major construction project to be awarded in the £10bn project.
EDF will invest in a £15m training campus in nearby Bridgwater.
The on-going discussions over the bailout to Greece will also be briefly discussed but Britain is not part of the Eurogroup which is currently assessing Athens' suitability for an extra 130bn euro (£108bn) bailout.
On Thursday, British and French troops trained together in Northumberland as the two countries adopt the closest military alliance in their joint history.
It is reported by French newspaper Les Echos that the men will announce a new partnership which would see the French-based Dassault defence contractor work with the UK's BAE systems on a prototype combat drone.
The two have been engaged in a furious bidding war to secure a lucrative contract with India. The French secured "preferred bidder status" after pitching their Rafale fighter jet.
The announcement will be symbolic, after tensions between the two countries over Britain's withdrawal from a fiscal compact to enforce discipline in the eurozone, and a surprise downgrade of France's credit rating.
There is also a political dimension for the French leader. Mr Sarkozy announced this week that he will stand again in the presidential elections this spring.
He is lagging severely in the polls and after appearing to secure the endorsement of the German chancellor, he needs all the high profile friends he can get.
