IS militants take control of last Iraq-Syria crossing
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 SOHR said.
The militants dominate the provinces of Deir al-Zour and Raqqa and have a strong presence in Hasakeh, Aleppo, Homs and Hama.
However, BBC correspondents say there are large areas under IS control in the east that are not very significant strategically.
IS has also been made significant gains in neighbouring Iraq, capturing the strategically-important city of Ramadi in Anbar province after weeks of fighting.
BBC's Jim Muir says that the militants, demonstrating the extraordinary momentum they have shown in the past, have not been content to sit and savour the major gains they have made in recent days.
The seizure of al-Tanf, in Syria's Homs province, enables IS to link up its positions in east-central Syria more directly with the ground they hold in Iraq's western Anbar province, he notes.
Militants in Iraq are reported to be pressing eastwards from Ramadi down the Euphrates Valley towards Habbaniya where pro-government forces are massing for a proposed counter-attack on 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.
"There's no doubt there was a tactical setback, although Ramadi had been vulnerable for a very long time," he told The Atlantic magazine in an interview published on Thursday.
"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."
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 IS in their own country," he said, "this is going to continue to be a difficult challenge."
The US and its coalition partners have carried out 18 air strikes against Islamic State targets in Syria and Iraq since Wednesday, according to a statement from the Combined Joint Task Force.
Eight of the strikes in Syria targeted tactical units, a staging area and trench system, as well as fighting positions and vehicles near Al Hasakah, Aleppo, Ar Raqqah and Kobane, the statement said.
Meanwhile, the United Nations has expressed concern at reports that Syrian forces in Palmyra prevented civilians from leaving, ahead of its fall to Islamic State.
It said it was "deeply concerned" about the plight of those remaining in the area, 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.
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