ISIS militants have reportedly captured the Syrian town of Maheen, in central Homs Province, from government forces.
The fighters launched the offensive with two suicide car blasts on October 31, the UK-based Syrian Observatory for Human Rights says.
Clashes were also taking place in nearby Sadad, a mostly-Christian town.
The latest development comes amid air campaigns in Syria by Russia and a US-led coalition.
ISIS has been expanding from its mainly northern and eastern strongholds towards Homs in central Syria in recent months. The group overran the town of Tadmur – home to the ancient ruins of Palmyra – and al-Qaryatain town.
The Observatory said at least 50 government soldiers were killed or wounded in the fighting. The attack on Maheen began late on October 31 with twin suicide car bombs, a favored tactic for ISIS militants launching an assault.
By November 1 the Observatory reported that the whole town was reported to be in ISIS hands. An ISIS statement also said the group had taken Maheen.
Maheen is home to a large military complex and arms depot.
Meanwhile, clashes between government troops and ISIS are said to be continuing on the outskirts of Sadad. The town is home to Syria’s Assyrian Christian minority, where the ancient language of Aramaic is still spoken.
It comes amid continued Russian air strikes in support of Syrian President Bashar al-Assad, which Russian officials say are targeting ISIS and other “terrorist groups”.
However, activists on the ground say the strikes have been hitting moderate rebels and civilians in western areas, where ISIS have little or no presence.
They said more than 60 people were killed by Syrian army raids and Russian strikes in the northern province of Aleppo on October 31.
On October 30, more than 70 people were reported killed and hundreds more wounded in an air strike and shelling on a market in the rebel-held Damascus suburb of Douma.
In an attempt to ward off the attacks, rebel groups in Douma are reportedly using captured soldiers and other people associated with the government as human shields.
The US-led coalition, which is also hitting ISIS targets in Syria, said on November 1 it had conducted nine air strikes across the country, including in Mar’a and al-Hawl, in the north.
This week the White House announced that fewer than 50 US special forces troops would be sent to Syria to assist anti-government rebels in fighting ISIS.
Separately on November 1, Syrian Foreign Minister Walid Muallem met UN peace envoy Staffan de Mistura in Damascus to discuss ongoing international talks on the Syria conflict.