EU foreign policy chief Catherine Ashton urged Syria on Thursay to implement "fully and rapidly" an Arab League plan to end a months of bloodshed.
"I welcome the efforts of the Arab League to end violence and bring about the reforms that the Syrian people have bravely demanded these last seven months," she said in a statement.
"It is now vital that the commitments made by the Syrian authorities to the Arab League are followed through fully and rapidly, under Arab scrutiny."
Ashton added that the initiative could only succeed, however "if the Syrian authorities provide the space and security for opposition groups to work with all sectors of the Syrian population for a peaceful political transition."
Syrian troops killed 20 civilians and arrested dozens on Thursday, a rights watchdog said, a day after Damascus pledged to withdraw its forces from protest hubs under the deal with the Arab League.
The peace plan also calls on Syrian President Bashar al-Assad to engage in a national dialogue with his opponents.
But Assad's opponents are sceptical about his readiness to rein in a brutal crackdown that the United Nations says has cost more than 3,000 lives since mid-March.