Syrian troops have reached the edge of the northern province of Raqqa, home to the de facto capital of the Islamic State group's self-styled caliphate.

The government has had no presence in Raqqa since August 2014, when IS captured the Tabqa air base and killed scores of captured government soldiers.

The Britain-based Syrian Observatory for Human Rights says Syrian troops reached the "administrative border" of Raqqa province under the cover of Russian airstrikes.

The media arm of Lebanon's Hezbollah group, which is fighting alongside government forces, says Syrian troops reached the border of Raqqa province Saturday after advancing about six kilometers (4 miles) in the past hours.

Syrian troops launched an offensive toward the province on Wednesday.

IS is based in the provincial capital, also called Raqqa.