BRUSSELS—The European Union has praised moves by Syria’s opposition groups to establish a united front, and urged President President Bashar Assad on Sunday step aside to allow a political transition to take place.

The bloc also hailed Libya’s declaration of liberation, which formally ended the eight-month civil war against Moammar Gadhafi’s regime.

“The Syrian people must be able to define the future of their country without the fear of repression,” said a statement after a summit of the bloc’s 27 leaders.

Unlike Libya’s National Transitional Council, which brought together most factions fighting Gadhafi’s regime and was quickly recognized by much of the international community, Syria’s opposition still has no clear leadership. Last month, a 140-member Syrian National Council was established in an attempt to unify the fragmented opposition to Assad’s regime.

“The European Council welcomes the efforts of the political opposition to establish a united platform. The creation of the Syrian National Council is a positive step forward,” the statement said.

Still, officials have said the opposition needs more work to become an effective political force and to gain formal recognition as a legal representative of the Syrian people.

The uprising against Assad began in mid-March amid a wave of anti-government protests in the Arab world that toppled autocrats in Tunisia, Egypt and Libya. Assad has reacted with deadly force that the UN estimates has killed more than 3,000 people.

The EU has already imposed two rounds of sanctions against Assad’s regime. Leaders said a third may be necessary soon.

“We’ve said that in Syria President Assad should step down,” Britain’s Prime Minister David Cameron said. “I’ve made clear today there will be further sanctions — including through the EU — if the repression does not end.”

The European bloc already has banned investment in Syria’s oil sector and forbidden EU-based operators from participating in joint ventures with Syrian companies or providing credits and loans. It also has imposed travel bans and frozen the assets of individuals linked to Assad’s regime.

The EU has urged all members of the UN Security Council “to assume their responsibilities with relation to the situation in Syria.”

Earlier this month Russia and China vetoed a U.N. Security Council resolution condemning the bloodshed in Syria. They have argued that NATO misused a previous UN measure authorizing the use of force to protect civilians in Libya to justify months of air strikes and to promote regime change.

They expressed fears that any new resolution against Syria might be used as a pretext for a similar armed intervention.