Russian-backed Syrian government forces will enter the Islamic State-held city of Palmyra "very soon", a Syrian military source said on Wednesday, as government forces seek to win back the city from the group for the second time in a year.

The army said on Wednesday it had captured an area known as the "Palmyra triangle" a few kilometers (miles) west of the city.

Backed by Russian air strikes, the Syrian army has advanced to the outskirts of Palmyra in the last few days. "The army's entry to the city will begin very soon," the military source told Reuters.

The Syrian government lost control of Palmyra to Islamic State in December, having first recaptured it with Russian air support last March. The group has razed ancient monuments during both of its spells in control of the UNESCO World Heritage Site - destruction the United Nations has condemned as a war crime.

The Syrian Observatory for Human Rights, a UK-based organization that reports on the war, said government forces were expected to storm Palmyra at "any moment". Russia has said its aircraft are supporting the army offensive in Palmyra.

Photos published on an Islamic State Telegram account on Wednesday showed the group's fighters firing at the Syrian army with rockets and a tank. Reuters could not verify the authenticity of the photos.

Islamic State first captured Palmyra from the government in 2015. During its first period in control of the site, the jihadists destroyed monuments including a 1,800-year-old monumental arch.

Most recently, Islamic State has razed the landmark Tetrapylon and the facade of Palmyra's Roman Theater. Palmyra, known in Arabic as Tadmur, stood at the crossroads of the ancient world.

The government and its allies lost Palmyra as they focused on defeating Syrian rebel groups in eastern Aleppo. The rebel groups were driven from eastern Aleppo in December, the government's biggest victory of the war.