Islamic State of Iraq and the Levant (Isil) has seized the last border crossing between Syria and Iraq controlled by the Syrian government after security forces withdrew, the Syrian Observatory for Human Rights monitoring group said yesterday.
Iraqi officials said Iraqi security forces had also withdrawn from their side of the crossing, known as al-Waleed in Iraq and al-Tanf in Syria.
The crossing is in Syria's Homs province, where Isil seized the historic city of Palmyra late on Wednesday.
Despite the sweeping gains, the Obama administration is publicly voicing confidence in Iraq's prime minister in the fight against the militants, but privately some US officials question whether he is too weak to bridge the sectarian divide.
Washington is still betting on Prime Minister Haider al-Abadi, who stands at the centre of President Barack Obama's strategy to roll back the latest conquests of Isil while keeping the US from being pulled deeper into a conflict US combat troops left in 2011.
But even if Abadi's forces manage to retake the provincial capital of Ramadi after its capture last weekend by Isil, his reliance on Iran-backed Shia militias could strengthen his political rivals in Baghdad and signify US acquiescence to greater Iranian influence in Iraq.
An Isil fighter confirmed the group had taken control of the crossing, which is 240km drive from Palmyra, known as Tadmur in Arabic.
Isil also controls a border crossing between Syrian province of Deir al-Zor and the Iraqi province of Anbar, while a border crossing between the two countries in north-eastern Syria is controlled by a Kurdish militia, the YPG.
Separately yesterday it emerged that US air strikes against Islamist extremists in Syria killed two children by mistake, marking the first time the US military acknowledged inflicting civilian casualties in the war. Lieutenant General James Terry, head of the US-led air campaign against Isil, said: "We regret the unintentional loss of lives."
The air strikes on November 5-6 last year were targeting members of the Khorasan group, an offshoot of al-Qa'ida, in Harim City, according to the conclusions of an inquiry.
"The strikes were designed to destroy targets utilised by Khorasan group-affiliated extremists to meet and manufacture explosives," the report said.
But the bombing killed two children - including the daughter of a militant - and caused "minor injuries" to two civilian workers who lived near the buildings that were targeted, it said.
In assessments prior to the air raid, there were no reports that children could be in the area, the probe concluded.
The report found no wrongdoing or negligence by military forces overseeing or carrying out the strikes.
Human rights groups have alleged dozens of civilians have been killed in various air strikes on extremists in Syria and Iraq.
But Pentagon officials had maintained previously that they could not confirm that any civilians had been killed in the air campaign, despite more than 4,000 bombing raids.
The air war was launched in Iraq in August and extended to Syria in September.
Military investigators had reviewed an account of the strike from the Syrian Network for Human Rights, statements from Syrian "civil society contacts" to the State Department and video from the strike, the inquiry report said.
Pentagon spokesman Commander Elissa Smith said: "It is important to note that the current environment in Syria and Iraq makes investigating these allegations extremely challenging.
"Traditional investigative methods, such as examining the site, are not typically available."