Syria's main opposition bloc and the Free Syrian Army (FSA) rebels have agreed to co-ordinate their action against President Bashar al-Assad.
The Syrian National Council (SNC) also said the FSA had agreed to cut back attacks on government forces, after their first meeting in southern Turkey.
Analysts say the FSA, formed by army deserters, had posed a dilemma to the bloc, which urges non-violent tactics.
Earlier, the EU tightened sanctions against Mr Assad's government.
Foreign ministers in Brussels added 11 entities and 12 people to its sanction list, an EU official said.
On Thursday, the UN said the death toll in Syria has risen to at least 4,000 people, with human rights chief Navi Pillay saying information suggested it was "much more".
Ms Pillay said the conflict could be classed as civil war.
The meeting in the southern Turkish province of Hatay was the first between SNC head Burhan Ghalioun and FSA chief Riyad al-Asaad since their respective organisations were formed earlier this year.
The SNC's Khaled Khoja said leaders "agreed that it would be a co-ordinated movement", AFP reports.
"The council recognised the Free Syrian Army as a reality, while the army recognised the council as the political representative" of the opposition, Mr Khoja added.
The BBC's Jonathan Head in Istanbul says this is an important declaration of common purpose by the two most prominent groups in the Syrian opposition movement.
The Syrian National Council was formed three months ago and aims to represent all those opposed to President Assad, who has been fighting to crush an uprising since March.
Mr Khoja added that an understanding had been reached in which the FSA agreed to use force only to protect civilians.
Last week, Mr Ghalioun had urged the FSA not to undertake "offensive actions against the army", following a number of attacks in recent weeks.
Turkish officials say they have been pressing both these groups to focus on stopping a slide into full-scale civil war, a goal that requires the Free Syrian Army to scale back its attacks, our correspondent says.
In Brussels, European ministers said Syrian repression risked taking the country down "a very dangerous path of violence, sectarian clashes and militarisation", according to AFP.
The EU imposed a 10th round of sanctions on the government, placing bans on exporting gas and oil industry equipment to Syria and trading Syrian government bonds.
It also expanded a blacklist of companies and individuals which face assets freezes and travel bans.
Meanwhile, Arab League chief Nabil Elaraby, who joined diplomats at the meeting in Brussels, said his organisation was not seeking foreign intervention.
The move by the EU came just days after sanctions were announced by the Arab League.
Syria says it is fighting an armed insurgency and accuses foreign countries of meddling in its affairs.
Earlier this week, the Syrian authorities said they have released 912 people detained for their involvement in protests, and that 1,700 prisoners were released earlier this month.