Government forces withdrew from al-Tanf - known as al-Waleed in Iraq - crossing as IS advanced, the Syrian Observatory for Human Rights (SOHR) said.
The loss of al-Tanf to IS follows the group's takeover of the ancient Syrian 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 UK-based SOHR said.
The militants dominate the provinces of Deir al-Zour and Raqqa and have a strong presence in Hasakeh, Aleppo, Homs and Hama. » | Friday, May 22, 2015