Islamic State (IS) has launched an offensive against government forces at several places in eastern Syria, including attacks on a hospital in Deir el-Zour, near the border with Iraq.
IS fighters captured several buildings, including the hospital, in clashes that left more than two dozen people dead on both sides.
Control of the city is split between the regime of President Bashar al Assad and IS.
Opposition activist Omar Abu Leila is from Deir el-Zour but currently lives in Europe. He said IS fighters entered the hospital and shot dead several police guards they had captured.
About 35 pro-government fighters were killed and some medical staff taken hostage. More than 20 IS fighters were also reported killed.
The UK-based Syrian Observatory for Human Rights, which monitors Syria's five-year-long war, reported that government forces retook the al-Assad hospital after several hours, but the fate of the hostages was unclear.
Correspondents said the latest offensive showed that despite recent setbacks, IS could still threaten large amounts of government territory.
IS controls more than of half of Deir al-Zour is seeking to capture the entire city. The group has been besieging government-held areas there for two years, trapping about 200,000 civilians.
The Syrian government and its Russian allies make regular aid drops into the encircled city and there are frequent air strikes on Islamic State targets in and around Deir al-Zor.
Deir al-Zour is in an oil-rich area and on a vital supply route to the IS stronghold of Raqqa.