An Iraqi suicide bomber handing out food and pastries blew himself up outside the mainly Shi’ite port city of Basra Saturday, killing at least 53 people and wounding 137 others.

Local authorities said the blast targeted Shi’ite worshippers taking part in a major religious commemoration, Arbaeen. It marks the 40th day since the slaying anniversary of one of Shi’ite Islam’s most revered figures, Imam Hussein.

There has been no claim of responsibility for Saturday’s blast. It is the latest in a series of attacks during Shi’ite religious commemorations that threaten to further increase sectarian tensions in the country, just weeks after the U.S. withdrawal.

The United Nations’ top envoy in Iraq, Martin Kobler, condemned the attack. He urged all Iraqis to maintain their shared values and break the “vicious” cycle of violence in the country.

Iraq’s government, headed by Shi’ite Prime Minister Nouri al-Maliki, is dominated by Shi’ites, but it includes Sunnis and Kurds in a fragile power-sharing system that is rife with sectarian tensions.

More than 100 people have been killed in a spate of attacks on Shi’ites since the government issued an arrest warrant for Sunni Vice President Tariq al-Hashemi last month. He is charged with operating a death squad. Hashemi, who fled to Iraq’s autonomous Kurdish region to avoid detention, has denied the charges.