"Syrian army units withdrew from all of the Maheen and Hawareen areas after an IS attack," said Rami Abdel Rahman, head of the Syrian Observatory for Human Rights.
The two areas had previously been held by IS, but in late November the army and pro-regime militias were able to recapture them.
Abdel Rahman said fierce fighting was raging between regime forces and IS in the hills around the two areas.
In nearby Sadad, a local representative said the army had deployed to secure an exit route for regime forces fighting the jihadists, "who are trying to encircle and isolate them".
The Christian town of Sadad, some 18 kilometres from Homs city in central Syria, had been preparing for the possibility of an IS assault before regime forces recaptured Maheen and Hawareen.
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Sadad has been a battlefield before, changing hands between regime and rebel forces several times in late 2013 before government troops finally secured control.
Elsewhere, the Britain-based Observatory said the toll in air strikes on the town of Hamouriyeh in Eastern Ghouta outside the capital Damascus had risen to 19 dead, including six children.
The monitor said it was unclear if the strikes were carried out by regime or Russian warplanes.
Eastern Ghouta is a rebel stronghold outside the capital and is regularly targeted by the regime.
Moscow began an aerial campaign in Syria in support of the government at the end of September.
More than 250,000 people have been killed in Syria since the conflict began in March 2011 with anti-government protests.