Palmyra: Syrian forces trapped civilians, UN says. May 22, 2015

The United Nations says it has received reports that Syrian forces in Palmyra prevented civilians from leaving, ahead of its fall to Islamic State militants.

The UN, though not present in Palmyra, cited "credible sources".

It said it was "deeply concerned" about the plight of civilians remaining in Palmyra, amid reports of summary executions.

IS has also overrun the World Heritage site adjacent to the modern city, raising concerns about its future.

The militants have previously demolished ancient sites that pre-date Islam.

UN cultural organisation Unesco says its destruction would be "an enormous loss to humanity", but no damage has been reported there yet.

Jump media playerMedia player helpOut of media player. Press enter to return or tab to continue.

Media caption

Historian Dan Cruickshank explains the significance of the ancient Syrian city of Palmyra

IS has also taken control of a military airbase and a notorious prison near to Palmyra.

Meanwhile, IS has seized the last border crossing between Syria and Iraq after Syrian government forces withdrew, according to the Syrian Observatory for Human Rights.

The loss of the al-Tanf crossing in Homs province means the Syrian government does not control any of the country's border posts with Iraq.

The fall of Palmyra comes just days after IS captured the major Iraqi city of Ramadi.

The US has acknowledged the militants' gains are a "setback" for coalition forces targeting IS, but President Barack Obama insisted the US was not losing the war with the group.

White House spokesman Josh Earnest said the problem of IS was "not going to be solved overnight".

"Until we're able to build up local forces on the ground in Syria who can take the fight to Isil [IS] in their own country," he said, "this is going to continue to be a difficult challenge."