At least six people have been killed and more than 20 wounded after a bomb exploded near a Shia Muslim shrine south of the Syrian capital Damascus a day ahead of Ashura – a day of mourning observed by Shia Muslims, according to the interior ministry.

The blast near the Sayeda Zeinab mausoleum, Syria’s most visited Shia pilgrimage site, was caused when a motorcycle exploded near a taxi, the interior ministry said on Thursday. It called it a “terrorist bombing”.

State television earlier reported that the explosion was caused by a “bomb placed in a taxi by unidentified people”.

Al Jazeera was not able to independently verify the casualty figures and the circumstances around the blast.

“We heard a huge blast and people began to run,” 39-year-old civil servant Ibrahim told AFP news agency.

“Then ambulances arrived and security forces cordoned off the area.”

He said the explosion took place “near a security building around 600 metres from the mausoleum of Sayeda Zeinab”, granddaughter of the Prophet Muhammad and the daughter of Imam Ali.

The London-based monitor Syrian Observatory for Human Rights said that the explosion took place close to positions of Iranian militias, a key ally of Syrian President Bashar al-Assad alongside Russia in Syria’s continuing conflict now in its 13th year.

Photos shared by Al-Ikhbariya state TV and pro-government media showed the charred taxi surrounded by large crowds of people and men in military fatigues. The area’s buildings had green, red, and black Ashura flags and banners hung.

In a video shared on social media, people carried two men covered in blood and dust off the ground while calling for help. The glass facades of shops nearby had shattered, while one was on fire.

Ashura is the 10th day of the Islamic month of Muharram, which is one of the holiest months for Shia Muslims. It marks the martyrdom of the Prophet Muhammad’s grandson, Imam Hussein, and his 72 companions in the battle of Karbala in the seventh century in present-day Iraq. Ashura marks the peak of the mourning procession.

Sunni Muslims commemorate the day through voluntary fasting as it marks the day Nuh (Noah) left the Ark and the day Musa (Moses) was saved from the pharaoh of Egypt by God.

They also emulate the Prophet Muhammad who used to fast on Ashura in Mecca, where it became a common tradition for the early Muslims.

The explosion is the second in the Sayeda Zeinab neighbourhood in the days leading to Ashura. On Tuesday, Syrian state media citing a police official said that two civilians were wounded after a motorcycle laced with explosives was detonated.

Earlier this week, two people were wounded in a separate blast outside the shrine, where pilgrims have been flocking to mark a mourning period for Shia Muslims.

The Sayeda Zeinab mausoleum was hit by several deadly bombings during the country’s civil war that erupted in 2011.

Since then, the mosque complex with its turquoise ceramics and gold dome in the Iranian style has been defended by Shia militiamen, mostly Lebanese and Iraqi, as well as by the army.

Attacks have nonetheless declined in recent years, with some of the security measures relaxed.

The Islamic State armed group (ISIL/ISIS) said it was behind a double suicide attack in February 2016, 400 meters from the mausoleum, that killed 134 people, including more than 90 civilians.

The group had also claimed a triple blast near the sanctuary several weeks earlier that took the lives of at least 70 people.