The Syrian army has lost control of the last border crossing between Syria and Iraq to Islamic State (Isis) militants on 22 May, a day after the group's takeover of the ancient central Syrian city of Palmyra.
The Syrian Observatory for Human Rights (SOHR) reported that government forces retreated from al-Tanf (known as al-Waleed in Iraq) as IS advanced. The crossing is located in the Homs province 150 miles from Palmyra and allows IS to connect its positions in east-central Syria with the Iraqi Anbar province.
The extremist group now controls "more than 95,000sq km (36,679sq mi)" which is more than half of the Syrian territory.
Palmyra was captured after clashes with Syrian government forces. IS fighters entered the ancient ruins of the 2,000-year-old world heritage site after seizing the city's military air base, prison, and intelligence headquarters.
The Islamist group claimed to be in complete control of the city after the "collapse" of pro-government forces. The retreating forces "left behind a large number of [their] dead", IS said on Twitter.
The military victory represents a serious setback for embattled Syrian president Bashar al-Assad, as Palmyra sits on a strategic route that connects the capital, Damascus, and the contested eastern city of Deir al-Zour. It is also the first time that IS has taken over a city directly controlled by the Syrian army.
The jihadist group has also conquered the strategic city of Ramadi in Anbar province after a long battle with the Iraqi army. However, US president Barack Obama insisted that America was "not losing" the war with the group.
"There's no doubt there was a tactical setback, although Ramadi had been vulnerable for a very long time," he told The Atlantic magazine. "The training of Iraqi security forces, the fortifications, the command-and-control systems are not happening fast enough in Anbar, in the Sunni parts of the country."