Islamic State militants have blown up two ancient shrines they consider sacrilegious in Palmyra, the jihadi group said on Tuesday.

The report was the first of any damage being done by the militants to buildings in Palmyra, a 2,000 Unesco world heritage site in central Syria, since they seized control of the city in May. Syrian forces have bombed the city, and the militants camped within it, since then.

Before and after pictures showed several militants carrying explosives and the shrines, which are not among the city’s monumental Roman-era buildings, reduced to rubble.

News of the destruction in Palmyra emerged as the chief spokesman of Isis issued a call for increased violence during the holy month of Ramadan, and called upon Sunnis in Iraq to rise up against what he described as Shia oppression intended to end the Sunni presence in the country.

Isis also released a video apparently showing the brutal killing of men alleged to be spies. In the recording, the men are drowned in a cage submerged in a swimming pool.

The video is similarly brutal to another released earlier this year showing the immolation of the Jordanian pilot Muath al-Kasasbeh who was taken prisoner after ejecting from his plane, which he was flying on a mission with the US-led anti-Isis coalition.

In a recording released on Tuesday, Isis spokesman Abu Mohammad al-Adnani called for intensified jihadist attacks during Ramadan.

He also called on Sunnis in Iraq to rise up against what he described as Shia oppression and discrimination, saying Sunnis were no longer allowed to practise their religion openly in government strongholds like Baghdad and were subject to violence and collective punishment.

He urged Sunnis to place their lot with Islamic State, saying the Shia-dominated government had betrayed those who stood by it, including those who joined the US-backed Anbar Awakening campaign and rebelled against the jihadists. He also said that the government-backed popular militia, the Hashd al-Shaabi, which is Shia-dominated, had abused Sunnis and destroyed their homes in villages and towns liberated from Isis and barred the displaced from returning to their homes.

“The Crusaders have decided to empty Iraq of its Sunnis,” said al-Adnani. “Wake up, Sunnis everywhere.”

The Isis spokesman also urged rebel groups in Syria not to fight Islamic State. The Hashd al-Shaabi has taken the lead in the battle against Isis in Iraq, playing a crucial role with direct Iranian support in ousting Isis from Saddam Hussein’s hometown of Tikrit earlier this year. But the militias have been accused of an array of human rights abuses and the alleged inflicting of collective punishment on villages taken from Isis.

While the terror group has scored important victories in recent months, wresting control of the historic city of Palmyra from Bashar al-Assad’s regime and Anbar’s provincial capital of Ramadi in Iraq within a week in May, it has also suffered setbacks.

The militants lost Tal Abyad, a crucial border crossing with Turkey, last week to an assault by Kurdish militiamen backed by coalition air strikes, abandoning the city and retreating to Raqqa, the self-proclaimed caliphate’s seat of power in Syria.