At least 50 people have been reported dead and scores wounded in blasts near the Shia shrine of Sayyida Zeinab, south of Syria's capital Damascus.
The shrine, which is highly revered by Shia muslims, has been targeted before, most recently in February last year. It happened as the government and opposition groups gathered in Geneva in a bid to start talks aimed at a political solution to the conflict.
The EU said the Islamic State group claimed the attack which was aimed at disrupting talks. Both the Syrian government and opposition are in Geneva but the talks have yet to begin. The main opposition group says the government must first meet key humanitarian demands.
US Secretary of State, John Kerry urged both sides to seize the opportunity to end the bloodshed.
Mr Kerry said there was "no military solution" to the spiralling crisis, which he warned could engulf the region if the tentative UN-sponsored negotiations fail as previous attempts have.
The UN envoy to Syria has scheduled separate discussions with both sides in Geneva on Monday.
Sunday's attacks near Sayyida Zeinab were carried out by two suicide bombers, but some witnesses spoke of three blasts.The shrine contains the grave of one of the Prophet Muhammad's grand-daughters. It continues to draw many Shia pilgrims, despite the civil war.