Syrian fighter planes raided areas close to the Iraq border under the control of the Sunni Islamist rebels even as the Iraqi front of Isis took control of more territory in the north-west of the country and intensified the battle for the biggest oil refinery in the country.
At least 16 people were killed and dozens injured as jets bombed residential areas in the town of Muhassan, which lies just over 100 km from the border with Iraq.
Meanwhile, militants from the Islamic State in Iraq and the Levant (Isis) seized a border post on the Iraq-Syria frontier, Reuters reported, citing security sources.
The Iraqi government, which is tottering under the heavy onslaught of the Isis forces that saw the quick disintegration of the US-trained armed forces, denied the outpost had fallen to the militants, but the fast-paced advance of the Sunni militia looked poised to erase colonial era borders between the two strife-torn countries.
Isis's capture of al-Qaim, a strategic town in a key supply route, will also pit the al-Qaeda- inspired militant outfit against rival Sunni factions that control vast swathes of land in the civil war-afflicted Syria's eastern regions.
The neighbouring Albukamal border post within Syrian borders has been under the control of the Syrian Sunni opposition outfit called the Nusra Front, which has fought with Iraq's Isis on numerous occasions in the past for regional hegemony.
Colonial era border erased
Analysts who monitor the developments in Syria and Iraq have said the latest developments point to the smashing of a century-old border lines between Iraq and Syria drawn by colonial powers Britain and France.
The head of the Syrian Observatory for Human Rights [SOHR] monitoring group, Rami Abdulrahman, said Isis had pushed the Nusra Front out from many areas of eastern Syria in the past few days.
Isis's capture of al-Qaim will help them marginalise rival factions of Sunni militancy and allow them to consolidate their grip on the area straddling the two countries where they aim to establish a hardline Sunni Islamic caliphate, according to Rami Abdulrahman, the head of the SOHR.
The Isis has become a more dreaded outfit in the last two weeks following their confiscation of weapons from fleeing Iraqi troops and the looting of millions of dollars from banks in Mosul, the second biggest city of Iraq which they took control of in the beginning of their latest campaign.
With civil war looming over Iraq, neighbouring Iran, a Shiite country, has thrown its weight behind the Nouri al-Maliki government in Baghdad and said it would not hesitate to send troops to Iraq to defend the Shiite shrines.
However, Saudi Arabia, which is ruled by Sunnis, has warned Tehran not to meddle in the internal affairs of Iraq.