BEIRUT, Lebanon — Islamic State militants swept into the historic desert city of Palmyra in central Syria on Wednesday, and by evening were in control of it, residents and the Syrian state news media said, a victory that gives them another strategically important prize five days after the group seized the Iraqi city of Ramadi.

Palmyra has extra resonance as home to some of the world’s most magnificent remnants of antiquity, as well as the grimmer modern landmark of Tadmur Prison, where Syrian dissidents have languished over the decades.

But for the fighters on the ground, the city of 50,000 people is significant because it sits among gas fields and astride a network of roads across the country’s central desert.

As they have swept across Syria and Iraq, Islamic State fighters have destroyed or damaged numerous ancient sites and sculptures, condemning them as idolatry in slickly produced recruitment films, even as they pillage and sell off more portable items to finance their activities. That has raised fears both locally and internationally that Palmyra, a United Nations world heritage site, could also be irrevocably damaged.