BEIRUT — Syrian government forces succeeded Saturday in their deepest push yet into the eastern suburbs of the capital Damascus, cutting off a major highway that made it hard to move between three parts of the rebel-held area, and inflicting a major blow to opposition fighters who have vowed not to surrender.

Syrian state media and an opposition monitoring group said government forces have laid siege on the rebel-held town of Harasta, cutting it off from the rest of the suburbs known as eastern Ghouta.

The Britain-based Syrian Observatory for Human Rights said government forces have also cut off the main highway that links northern Ghouta with its southern parts, isolating the town of Douma, the largest in the area.

The Syrian Central Military Media said troops captured the Kilani gas station on the main highway linking Douma with Harasta and those two areas with the rest of eastern Ghouta. Residents will have to rely on unpaved roads to move in eastern Ghouta but that is difficult because of the intensity of shelling and airstrikes, activists said.

“The regime has cut Ghouta into three parts by cutting the main roads linking them,” said the Observatory’s chief Rami Abdurrahman.

The government and its Russian backers, determined to wrest eastern Ghouta from rebel control after seven years of war, recently intensified the shelling and bombardment to clear the way for its troops to advance on the ground. Nearly 1,000 people have been killed in the past three weeks of relentless bombardment.

SCMM said troops Saturday reached the center of the town of Misraba, and opposition fighters are crumbling amid the offensive.