Regime forces backed by Russian warplanes launched an offensive on December 25 in Idlib province, the last in the country that remained fully outside the government's control. They have recaptured dozens of villages.
Idlib province is almost entirely controlled by Hayat Tahrir al-Sham, a jihadist outfit dominated by Al-Qaeda's former Syrian affiliate.
The Syrian Observatory for Human Rights, a Britain-based war monitor, said regime forces were able to enter Abu Duhur airbase under air cover and were engaged in fierce fighting with the jihadists.
The airport, which straddles the border between the provinces of Idlib and Aleppo, has been under the control of Hayat Tahrir al-Sham since September 2015.
If it is recaptured, the airport will be "the first military base in the hands of the regime in Idlib province," said Observatory director Rami Abdel Rahman.
Since losing the airbase in 2015 following a two-year jihadist siege, the Syrian government has only had control of two villages in Idlib province: Fua and Kafraya.
The regime's goal is to retake the southeast of the province in order to secure a route between Damascus and second city Aleppo, both under its control, according to the Observatory.
More than 340,000 people are estimated to have been killed in the Syrian war, which began in 2011 as the regime brutally crushed anti-government protests. Millions have been displaced.