PALMYRA, Syria (Reuters) - Islamic State militants retreating from Palmyra laid thousands of mines that they planned to set off simultaneously as the army moved in, a Syrian officer told Reuters in the ancient city after its recapture from the jihadist fighters.

Graffiti (R) sprayed by Islamic State militants which reads "We remain" is seen at theTemple of Bel in historic city of Palmyra, in Homs Governorate, Syria April 1, 2016. REUTERS/Omar Sanadiki

The officer said main streets and side roads in Palmyra had been rigged with explosives weighing up to 50 kg. More than 3,000 had already been safely detonated since government forces, backed by Russian jets, retook the city on Sunday, he said.

He did not say why the Islamic State fighters failed to set off the explosives before pulling out, but his assertion echoed comments from Syria’s antiquities chief, who said the militants intended to dynamite a greater area of the city’s 2,000-year-old ruins than they already had.

The officer, who did not give his name, said the bombs left behind were linked so they could go off together.

“All the government buildings are rigged in a network connected to the Daesh leadership headquarters,” he said, using an Arabic acronym for Islamic State. “The idea was that as we enter it would all go off at once, not just bomb by bomb. And there are a really huge number of bombs.”

Islamic State’s defeat in Palmyra was not only a significant military victory for President Bashar al-Assad, opening up the country’s central desert to government forces and their allies. It also cast the Syrian army as an effective fighting force against jihadists bent on cultural vandalism and wanton killing.

A military source told Reuters on Saturday troops had identified 45 bodies in a mass grave in Palmyra, including civilians and Syrian army members captured by Islamic State.

Parts of Palmyra have been cleared, including the road from Homs. But Syrian soldiers -- soon to be joined by Russian de-mining experts -- are still working on defusing or detonating explosives.

“We cannot leave them there. We are dealing with 90 percent of them by exploding them because they are buried firmly in the ground, cemented in the asphalt,” the officer said.

Civilians, most of whom fled before Syrian and allied forces began the offensive, cannot return until de-mining is complete.

Smoke could be seen rising from some parts of the modern, residential city of Palmyra, which lies next to the 2,000-year-old ruins, during a visit by journalists on Friday.

But few people were to be seen and no shops were open. Residential areas had been damaged and traces of explosions could be seen on the ground.

GRAFFITI ON RUINS

As well as a network of bombs, Islamic State left traces of their 10-month rule in Palmyra. Graffiti was evident on some of the ancient stones. “Remaining”, it read, part of the Islamic State motto of “remaining and expanding”.

On a stone among the remains of the Temple of Bel was scrawled: “No shooting without the permission of the Emir.”

A signpost on a road through the ruins, now bent at 90 degrees to the ground, tells travelers in the group’s trademark black and white colors to “Respect God”.

“When we show humility, God will deliver us,” the sign says, above a passage from the Koran which says: “God made you victorious at the (battle of) Badr when you were but a humble (force),” referring to a battle from the early Muslim era which the ultra-hardline group looks back to with reverence.

The ancient Roman amphitheatre, where Islamic State shot dead around 20 men as it took over the city last May, appears unscathed.

The Temple of Bel, an imposing monument before it was blown up last year, has been reduced to a couple of columns and a heap of rubble, although antiquities chief Maamoun Abdelkarim says it is not totally beyond repair.

Other structures blown up by Islamic State include Palmyra’s triumphal arch, three funerary towers and the temple of Baal Shamin. Before serious renovation can take place, officials say, the area needs to be made fully safe.

Moscow has sent de-miners to help with the clear-up, and Russian military servicemen will start defusing mines in Palmyra in a few days, Russian news agencies reported on Saturday, citing the defense ministry.

The first batch of specialists has left Russian airbase Khmeimim in western Syria. The convoy, consisting of more than 20 vehicles, will be guarded by Mi-24 and Mi-28 helicopters.

The de-miners will deal with more than 180 hectares of territory, Russia’s defense ministry has said, citing initial estimates. The aim is to clear the historic part of the ancient city as well as residential areas.