Islamic State militants have rounded up and executed nearly 300 pro-government troops in just two days after capturing the Syrian city of Palmyra.

The jihadis have been stalking homes and shops looking for survivors, using loudspeakers to warn residents not to shelter any troops.

Many locals have come forward to give information about any soldiers who had melted into the civilian population.

Video posted on a pro-ISIS Facebook page showed people and militants gathering around two bloodied men in military uniforms in a Palmyra street.

'Let all the residents see them,' one of the men in the gathering tells an ISIS fighter.

Meanwhile, in Iraq, ISIS seized another town in Anbar province less than a week after capturing the provincial capital Ramadi, a tribal leader said today.

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Sickening: A picture sent by activists to a Western journalist showed a row of men in a street in Palmyra — known locally by its Arabic name Tadmur — lying in a pool of blood. At least four had been decapitated

Carnage: This picture released on a pro-ISIS website shows damaged Syrian military helicopters at Palmyra air base that was captured by the Islamic State militants after a battle with the Syrian government forces

Maamoun Abdulkarim, the head of the Antiquities and Museum Department in the Syrian capital Damascus, said: 'There are arrests and liquidations in Palmyra.'

He added that ISIS fighters are 'moving in residential areas, terrifying people and taking revenge.'

He added that many were shot dead in the streets.

Homs-based activist Bebars al-Talawy and an opposition Facebook page said that as many as 280 soldiers and pro-government forces have been killed in Palmyra since it was captured Wednesday.

The city is home to one of the Middle East's most famous archaeological sites.

But Abdulkarim said no gunmen were seen in the area of Palmyra's 2,000-year-old ruins, which once attracted thousands of tourists.

The Britain-based Syrian Observatory for Human Rights said ISIS fighters have killed 17 men in Palmyra and that it has unconfirmed reports of the killing of dozens more.

An ISIS militant rips down a poster of Syrian President Bashar al-Assad after raiding a compound in Palmyra

Jail break: This picture shared on several pro-ISIS Twitter account claims to show prisoners celebrating with their liberators after being freed from the notorious Tadmur prison by ISIS fighters in the city of Palmyra

Governor Talal Barazi of the central province of Homs, which includes Palmyra, said that ISIS fighters have abducted men and 'might have committed massacres.'

The Observatory and al-Talawy said ISIS's next target appears to be the Tayfour air base near Palmyra, where many of the Palmyra troops had retreated.

They said ISIS is sending reinforcement to the air base area.

In Iraq, meanwhile, tribal leader Sheikh Rafie al-Fahdawi said the small Iraqi town of Husseiba fell to ISIS overnight when police and tribal fighters withdrew after running out of ammunition.

'We have not received any assistance from the government. Our men fought to the last bullet and several of them were killed,' he said.

Husseiba is about four miles east of Ramadi, where ISIS militants routed Iraqi forces last weekend in their most significant advance in nearly a year.

Archaeological jewel: The city is home to a UNESCO World Heritage site. Jihadi fighters have been stalking homes and shops looking for survivors, using loudspeakers to warn residents not to shelter any troops

Al-Fahdawi said that with the fall of Husseiba, the militants have come closer to the strategic Habbaniyah military base, which is still held by government forces.

'The situation is very critical. The militants are about five kilometres from Habbaniyah base, which is now in great danger,' he said.

A day earlier, ISIS militants captured the Iraqi side of a key border crossing with Syria after Iraqi government forces pulled out.

The fall of the al-Walid crossing, also in Anbar, will help the militants to shuttle weaponry and reinforcements more easily across the Iraqi-Syrian border.

The Iraqi government plans to launch a counter-offensive in Anbar involving Iranian-backed Shi'ite militias, which have played a key role in pushing back the ISIS group elsewhere in the country.