Islamic State militants lost a vital supply route in eastern Syria on Monday with the destruction of the last bridge in the city of Deir Ezzor, a monitoring group said.
The regime of President Bashar al-Assad is believed to have caused the explosion which hit the Siyassiyeh crossing over the Euphrates river, the Syrian Observatory for Human Rights said.
Half of Deir Ezzor city is under IS control, while the other half is in the hands of forces loyal to Assad.
"All the signs seem to indicate that the (Assad) regime was responsible for the explosion," said Observatory director Rami Abdel Rahman.
The blast left IS-held parts of Deir Ezzor "completely under siege", said Abdel Rahman, who warned that tens of thousands of civilians had become trapped in IS-controlled neighbourhoods.
The city's other three bridges had already been blown up, by the regime and by rebels, at various stages of the three-year Syrian conflict.
An activist from Deir Ezzor, Mohammad al-Khleif, said that it had become "extremely hard" to move around the city.
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"Fighters and civilians alike will now have to use small boats to get across the Euphrates," he told AFP via the Internet.
Khleif said two other bridges had been blown up by regime troops trying to cut off rebel supply lines -- prior to the emergence of the IS -- while one had been destroyed by rebels seeking Assad's ouster.
Assad's regime has in recent weeks stepped up its fight against the IS, although a US-led coalition says it will not coordinate with Damascus in its own anti-jihadist campaign.
Elsewhere in the war-torn country, regime bombardment on rebel areas of the northern city of Aleppo killed at least 10 civilians, the Observatory said.
Aleppo has been under an intense aerial bombing campaign since December, defying a February UN Security Council resolution condemning such strikes.
And in Talbisseh, near the central city of Homs, another 15 civilians including three children were killed in barrel bomb attacks, the Observatory said.
Syria's conflict began as a popular revolt demanding Assad's ouster, but morphed into a brutal civil war after the regime unleashed a massive crackdown against dissent.
Months into the fighting, jihadists began streaming into Syria, and the IS now controls about a quarter of the country's territory.