The US has threatened Syria with "firm and appropriate measures" as the Syrian army reportedly prepares to retake a strategic province on the border with Jordan and the Israeli-occupied Golan Heights.

In a statement released on Friday, US State Department spokeswoman Heather Nauert expressed concerns about the upcoming operation in southwestern Dara'a province, claiming that it falls within a de-escalation zone in Syria.

“As a guarantor of this de-escalation area with Russia and Jordan, the United States will take firm and appropriate measures in response to Assad regime violations," she said.

The warning came two days after the so-called Syrian Observatory for Human Rights (SOHR) reported that the Syrian troops were moving into Dara'a after liberating all remaining militant-held areas near the capital, Damascus.

On Friday, Syrian state-run media reported that government aircraft had dropped leaflets in terrorist-controlled areas of Dara'a, urging foreign-backed militants to disarm.

One of the leaflets declares “the arrival of the Syrian Arab army’s soldiers," according to SOHR, which is sympathetic to foreign-backed militants.

The UK-based monitor also said the Syrian government had sent reinforcements to Dara'a following the completion of operations near Damascus.

“These forces are now stationed on the edges of Dara'a province,” SOHR head Rami Abdel Rahman said. “The goal is a broad offensive, should the rebels reject a negotiated pullout as was the case in Eastern Ghouta."

The recapture of Dara'a is highly important because it borders the occupied Golan Heights which Israel has used to treat wounded militants for years.

The territory's return to the Syrian government control would cut the much-reported collaboration between Israel and militants and deal a blow to Tel Aviv's plans to annex the Golan Heights.

Syrian army advances are also upsetting to US plans in the Arab country where it has deployed about 2,000 troops to carve out a statelet in the country's north with the help of Kurdish militants.

With Syria's military gains gathering momentum, the US has stepped up its attacks on army positions under numerous pretexts.

On Thursday, Syrian state media reported that the US struck Syrian army positions in eastern Syria, but the US military denied knowledge of it.

“Some of our military sites between Albu Kamal and Humeima were exposed at dawn today to aggression launched by US coalition jets,” state news agency SANA reported, citing a military source.

SANA said the strikes came within 24 hours of a Daesh attack on Syrian army positions in the same region, where the Takfiri terrorists are fighting government forces to the west of the Euphrates.

The Syrian army managed to retake the Eastern Ghouta region, on the outskirts of the capital Damascus, late in March.

On Monday, the General Command of the Syrian Army and Armed Forces said complete security had been restored to Damascus and its countryside after al-Hajar al-Aswad district and al-Yarmouk camp were totally purged of Daesh terrorists.