The Syrian government and the opposition have blamed each other for yesterday's explosion in the country's capital, Damascus.
At least 11 people died and 28 were injured in the blast, which went off as worshippers were leaving Friday prayers.
State television said the attack was the work of a suicide bomber, but opposition activists claim it was carried out by the authorities.
The European Union has said it is "extremely concerned" about continuing violence in Syria.
The trouble is in violation of a ceasefire and in spite of the presence of UN monitors on the ground.
"I call upon all parties in Syria to cease immediately all forms of violence," said EU foreign policy chief Catherine Ashton in a statement.
"I am extremely concerned about the continued violence in Syria in violation of the ceasefire which should have come into effect on 12 April and despite the presence on the ground of UN observers."
Ms Ashton said it was "clear that the Syrian government is not fulfilling its obligations and is failing to meet its commitments to withdraw its troops and heavy weapons from population centres.
"Therefore, I urge once more the Syrian authorities to fully respect the terms of the agreement with the UN and immediately and fully comply with (UN and Arab League envoy) Kofi Annan's plan endorsed by UNSC resolutions 2042 and 2043."
Ms Ashton said the Syrian government "must ensure that the ceasefire holds.
"It must also release detainees, provide access for journalists, ensure freedom to demonstrate peacefully and allow humanitarian assistance. I am very concerned at reports suggesting that the numbers of detainees have sharply increased since the agreement of the ceasefire.
She added: "It is crucial that all efforts are made to expedite the deployment of the UN observation mission to Syria (UNSMIS).
"I reiterate the EU's and (27) Member States' readiness to provide support that might be requested to guarantee the success of the observer mission."
A spokesman for Ms Ashton said earlier that the Syrian authorities were failing to respect their ceasefire pledge to withdraw troops from urban centres in accordance with the agreed peace plan.
Amnesty International said it had received the names of 362 people reportedly killed in Syria since UN observers deployed last week to monitor the peace deal.
The United States, which has repeatedly voiced fear that President Bashar al-Assad would use the official truce to attack the opposition, said it was ready to consider an end to the UN monitoring mission before its initial 90-day period is over.
Syria has "failed to meet its objectives because Assad isn't living up to his half of the bargain", said State Department spokeswoman Victoria Nuland.
Also yesterday, the UN put Major General Robert Mood of Norway, a veteran of troublesome truces, in charge of the force monitoring the faltering ceasefire.
More than 9,000 people have died since a popular uprising erupted against Assad's regime in March 2011, the UN says, while non-governmental groups put the figure at more than 11,100.
Reports from Syria cannot be independently verified as state authorities have barred international journalists and rights groups.
