Fierce fighting between IS and government forces has taken hold near Palmyra since 13 May
Militants from the Islamic State group took control of almost all of Syria's historic city of Palmyra on Wednesday, a monitor said.
Rami Abdel Rahman, head of the Syrian Observatory for Human Rights, said on Wednesday night that troops loyal to President Bashar al-Assad had withdrawn completely from much of the town in the face of the IS advance.
IS's new advance comes after fierce clashes with Syrian government forces on Wednesday at the northern and eastern edges of Palmyra.
IS had previously seized most of the northern neighbourhoods of Palmyra, the modern town also known as Tadmor, on Saturday, but was pushed out by government forces less than 24 hours later.
Fighting on Wednesday raged near security buildings in northern Palmyra, and close to the city's notorious prison in the east.
The IS offensive on the ancient world heritage site of Palmyra began on 13 May, and has since left more than 350 people dead.
Mohammad, an activist originally from the modern town, told AFP that the city was suffering from water shortages and intermittent electricity.
"A large number of people from the city's north have been displaced into other neighbourhoods. Some are sleeping in the streets," he said.
Activists in the town said on Wednesday that residents had been prevented from entering the government-held hospital for seven consecutive days.
On Monday, IS seized two gas fields north of Palmyra that the government had been using to generate electricity for areas under its control.
Antiquities officials fear that IS wants to destroy Palmyra's pre-Islamic cultural treasures, which include colonnaded streets and ancient citadels.
The city is also strategically located at the crossroads of key highways leading west to Damascus and Homs, and east to Iraq.