Syrian State TV has said that fighters from the so-called Islamic State have killed at least 400 civilians in the recently captured historic city of Palmyra.
The broadcaster said its information came from residents inside the beleaguered city.
Former Head of the British Army, Lord Dannatt, has called on the Government to "think the previously unthinkable" and consider sending ground troops to combat Islamic State (IS) in Syria and Iraq.
Writing in the Daily Mail, Lord Dannatt said he felt air strikes had "failed" to stop the advance of the extremist organisation.
He called for a "public and political debate to begin immediately" on the issue of deploying up to 5,000 infantry soldiers.
In light of this terrifying scenario, how much longer can Britain and the US continue to show such a lack of commitment to defeating IS mililtarily? Their default option of air strikes and limited assistance to indigenous forces has failed thus far.
We have now reached a point when we must think the previously unthinkable and consider that British troops, acting as part of an international coalition, may be required to mount a ground campaign in Iraq and Syria.
I am no gung-ho general who says 'just send the boys in and don't worry about the body bags', far from it, but faced with such a lethal and uncompromising enemy as IS - and with the lack of political and diplomatic solutions at our disposal - we can no longer rule out 'boots on the ground'.
IS has recently made gains in the Middle East by gaining control of the ancient town of Palmyra, in Syria, and the Iraqi city of Ramadi.
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The Islamic State has seized Syria's last border crossing with Iraq after Syrian government forces withdrew, a monitoring group said today.
The Syrian Observatory for Human Rights said that the militant group has taken control of the al-Tanf crossing, known as al-Waleed in Iraq.
The crossing is in Syria's Homs province, 150 miles from the historic city of Palmyra, the historic city IS seized from government forces.
Islamic State militants have killed at least 17 people, some of whom were beheaded, as it seized Palmyra, a monitoring group said today.
Members of the Syrian security forces and pro-government civilians in the World Heritage city were among those killed, the British-based Syrian Observatory for Human Rights reported.
The Observatory says Islamic State killed a total of 49 people in areas near Palmyra as it advanced towards the ancient city.
Tens of thousands of people living in Palmyra may have fled in the last few days during fighting between government forces and Islamic State militants.
Around a third of 200,000 Syrians living in the neighbourhood of the ancient city, a UNESCO World Heritage site, may have left the UN human rights office said today.
U.N. human rights spokeswoman Ravina Shamdasani in Geneva also said there were reports of government forces preventing civilians leaving until they themselves fled and Islamic State, also known as ISIS or ISIL, took control of the city.
ISIL has reportedly been carrying out door-to-door searches in the city, looking for people affiliated with the government.
At least 14 civilians are reported to have been executed by ISIL in Palmyra this week.
Smoke was rising above ancient remains in Palmyra in central Syria days before Islamic State militants announced they had seized full control of the historic city.
The radical group said in a statement posted by followers on Twitter today it was in full charge of Palmyra, increasing fears that Roman-era temples, colonnades and a theatre in the World Heritage site will be destroyed.
In February, Islamist militants in northern Iraq destroyed a priceless collection of statues and sculptures from the ancient Assyrian era.
British-based Syrian Observatory for Human Rights said Islamic State now controls more than half of Syrian territory following four years of civil war.
Clashes in the area since Wednesday (May 20), killed at least 100 pro-government fighters, said Rami Abdulrahman, head of the Syrian Observatory for Human Rights.
Islamic State says it is in complete control of the ancient Syrian city of Palmyra.
The militant group said in a statement on Twitter that pro-government defences had collapsed and fighters had now seized the military airport and prison.
A statement said the the retreating forces "left behind a large number of (their) dead" which it said filled the city square.
Earlier, a monitoring group said IS fighters had entered the ancient ruins of Palmyra amid growing fears the group could destroy historical artefacts.
Islamic State militants have entered the ancient ruins of Palmyra after taking control of the Syrian city, monitors say.
Insurgents swept into Palmyra's military air base, prison and intelligence headquarters on Wednesday and were now in the city's ancient sites, Rami Abdulrahman, head of the Syrian Observatory for Human Rights, said.
However, he said there were so far no reports of destruction of the artefacts so far.
There are fears the group will destroy the historical ruins, which are regarded as "one of the most important cultural centres of the ancient world".
Mr Abdulrahman said around 100 pro-Assad fighters had been killed in clashes since Wednesday and forces were now withdrawing.