BEIRUT — Syrian government forces advanced deeper into the southern province of Daraa Sunday under the cover of airstrikes, entering new villages amid reports that the United States has told rebels not to expect an American intervention to defend them from the army’s offensive.

Syrian government forces have been on the offensive in eastern parts of Daraa province for the past five days in an operation that aims to reach a main border crossing point with Jordan.

The push comes two months after government forces regained control of rebel-held eastern suburbs of the capital, Damascus, that freed thousands of government soldiers for the new battle in Daraa province.

Opposition activists have been reporting for days that President Bashar Assad’s government has sent some of its elite units to the south, including the Republican Guards and the Tiger force, which is commanded by Brigadier General Suheil al-Hassan.

The Britain-based Syrian Observatory for Human Rights and Daraa-based opposition activist Osama Hourani said that the United States has informed rebel groups in southern Syria that Washington will not intervene in the operations to defend them against the government offensive that is ongoing under the cover of Russian airstrikes.

Two US officials discounted the claim. They spoke on condition of anonymity to discuss internal deliberations.

Denying the rebels support would be a reversal from earlier pronouncements. The United States warned last month it would take ‘‘firm and appropriate measures’’ to protect a cease-fire in southern Syria.

Associated Press