BEIRUT -- Islamic State militants have destroyed parts of the second-century Roman amphitheatre and an iconic monument known as the Tetrapylon in Syria's historic town of Palmyra, the government and experts said Friday.

It was the extremist group's latest attack on world heritage, an act that the U.N. cultural agency called a "war crime." A Syrian government official said he feared for the remaining antiquities in Palmyra, which IS recaptured last month.

Also on Friday, Turkey's military said IS killed five Turkish soldiers and wounded nine in a bomb attack in northern Syria.

Turkey is leading Syrian opposition fighters in an offensive against the IS-held town of al-Bab in the Aleppo province, a push that has been bogged down since mid-November. Since its military intervention, Turkey has lost 54 soldiers in Syria, most of them in the al-Bab offensive.

After suffering several setbacks in Syria, IS has gone on the offensive-- reclaiming ancient Palmyra in December and launching an attack on a government-held city and military air base in Deir el-Zour in eastern Syria.

On Friday, the state news agency SANA said seven civilians were killed when IS shelled a residential area in the city of Deir el-Zour.

However, IS remains under pressure in northern Syria from Turkey and U.S-backed Kurdish forces, as well as in neighbouring Iraq where Iraqi troops backed by the U.S.-led coalition is fighting to retake the city of Mosul from the militants.

Palmyra, a UNESCO world heritage site that once linked Persia, India, China with the Roman empire and the Mediterranean area, has already seen destruction at the hands of the Islamic State group. The ancient town first fell to IS militants in May 2015, when they held it for 10 months. During that time, IS damaged a number of its relics and eventually emptied it of most of its residents, causing an international outcry.

Palmyra fell again to the group last month, only nine months after a Russian-backed Syrian government offensive was hailed as a significant victory for Damascus.

On Friday, Maamoun Abdulkarim, the head of Syria's antiquities department, said reports of the recent destruction first trickled out of the IS-held town late in December. But satellite images of the damage only became available late Thursday, confirming the destruction.

Abdulkarim said militants have destroyed the facade of the second-century theatre, along with the Roman-era Tetrapylon -- a set of four monuments with four columns each standing at the centre of the colonnaded road leading to the theatre.

Satellite imagery obtained by the Boston-based American Schools of Oriental Research, or ASOR, show extensive damage to the Tetrapylon. DigitalGlobe satellite imagery also shows damage to the theatre facade.

ASOR said the damage was likely caused by intentional destruction from IS, but the organization was unable to verify the exact cause.

IS extremists have destroyed ancient sites across their self-styled Islamic caliphate in Syria and Iraq, perceiving them as monuments to idolatry.

UNESCO's director-general, Irina Bokova, said the new destruction in Palmyra amounted to a war crime.

"The Tetrapylon was an architectural symbol of the spirit of the encounter and openness of Palmyra - and this is also one of the reasons why it has been destroyed," she said in a statement.

Abdulkarim told The Associated Press that only two of the 16 columns of the Tetrapylon remain standing.

The Palmyra Tetrapylon, characterized by its four plinths that are not connected overhead, had only one original ancient column, said Abdulkarim. The 15 other columns were modelled after the ancient one and installed by Palmyra's 81-year old distinguished antiquities scholar Khaled al-Asaad, who was killed by IS militants when they were controlling the town the last time. The militants hung his body from a Roman column.

It was not immediately clear if the original column survived the destruction, Abdulkarim said.

ASOR said new stone debris was scattered across the centre stage from damage to the stage backdrop that is also the facade of the theatre.

During their first stay in Palmyra, IS destroyed ancient temples -- including the Temple of Bel, which dated back to A.D. 32, and the Temple of Baalshamin, a structure of stone blocks several stories high and fronted by six towering columns. The group also used the theatre for public killings and posted chilling videos of the slayings.

The militants also blew up the Arch of Triumph, built between A.D. 193 and A.D. 211.

Spokesman for Russian President Dmitry Peskov said Syrian troops are continuing their efforts to take back Palmyra. Peskov called the new destruction "barbaric," saying that it is a "real tragedy for the historic heritage."

On Friday, Syria's state news agency said government forces and allied troops have clashed with IS militants south of Palmyra, part of a new week-old offensive to reclaim the city.

Abdulkarim said he fears for what remains of the city's ancient relics.

"When Palmyra fell for the second time, we shed tears because we expected this terror," he said. "Now we are destined to see more terror if (IS control of Palmyra) continues."

Palmyra, with its 2,000-year-old towering Roman colonnades and priceless artifacts, was affectionately referred to by Syrians as the "Bride of the Desert."

A desert oasis surrounded by palm trees in central Syria, Palmyra is also a strategic crossroads linking the Syrian capital, Damascus, with the country's east and neighbouring Iraq. Located 215 kilometres (155 miles) east of Damascus, the city was once home to 65,000 people before the Syrian civil war began.

However, most Palmyra residents did not return after it was retaken by the government. Activists estimate the city is now home to a few hundred families. Many residents tried to flee as IS recaptured the city in December.

On Thursday, reports emerged that the militant group killed 12 captives it held in Palmyra, some of them beheaded in the Roman theatre.

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Associated Press writer Lori Hinnant in Paris and Vladimir Isachenkov in Moscow contributed to this report.