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Islamic State militants have seized the last Syrian government-controlled border crossing between Syria and Iraq, a monitoring group says, the BBC reports.

Government forces withdrew from the al-Tanf crossing in Homs as IS fighters advanced, the Syrian Observatory for Human Rights (SOHR) said.

The loss of al-Tanf to IS follows the group's takeover of the ancient city of Palmyra on Thursday.

The US says that fighting the militants will be a "difficult challenge."

Islamic State now controls "more than 95,000 sq km (36,679 sq miles)" of Syria, which is 50% of the country's entire territory, the SOHR said.

The militants dominate the provinces of Deir Ezzor and Raqqa and have a strong presence in Hasakeh, Aleppo, Homs and Hama.

However, correspondents say there are large areas under IS control in the east that are not very significant strategically.