BEIRUT (AP) -- Syrian government forces advanced to within six miles (10 kilometers) of the Islamic State-occupied Tabqa air base in the northern part of the country on Sunday, part of a push to try to unseat the extremist group from its de facto capital, Raqqa.
Government forces recaptured the nearby Thawra oil field from IS militants, according to a Syrian journalist Eyad al-Hosain, who is embedded with the army. The Britain-based Syrian Observatory for Human Rights monitoring group said the army advanced to within seven kilometers (four miles) of Tabqa.
The Tabqa base, 45 kilometers (28 miles) from Raqqa, holds strategic and symbolic value in the government campaign on the IS capital. It was the last position held by government forces in the Raqqa province before IS militants overran it in August 2014, killing scores of detained soldiers in a massacre they documented on video. Raqqa itself became the militants' first captive city.
A Syrian opposition coalition, meanwhile, called on Turkey to investigate the deaths of at least eight Syrian refugees, including four children, who were allegedly shot dead by border guards Saturday night while trying to cross the frontier.
A statement by the National Coalition of Syrian Revolution and Opposition Forces accused Turkish border guards of firing at a group of civilians trying to cross from Kherbet al-Jouz in northwestern Syria into Turkey's Hatay province, killing 11 people.
The coalition, which relies on Turkish political and financial support, said the incident "clashes with the generosity displayed by the Turkish government and brotherly people toward displaced civilians."
The Britain-based Syrian Observatory for Human Rights put the death toll at eight. The Local Coordination Committees, an activist network, said at least one of those trying to cross was from Jarablus, a northern Syrian town under Islamic State control.
A senior Turkish official said "we are unable to independently verify the claims" regarding the shooting, but said authorities were investigating.
"Turkey provides humanitarian assistance to displaced persons in northern Syria and follows an open-door policy — which means we admit refugees whose lives are under imminent threat," said the official, speaking on condition of anonymity in line with government regulations.
The Syrian war has pushed over 2.7 million refugees into Turkey, according to the United Nations. Turkey has tightened security along its border in recent months to prevent further inflows. The Observatory says border guards have shot dead 60 refugees trying to cross since the start of this year.
Meanwhile, Russia's defense ministry said a Russian soldier has died of wounds he suffered in a militant suicide attack in Syria, the military's 11th casualty since the start of its campaign there.
Sgt. Andrei Timoshenkov died at a hospital at the Russian base in Syria, the ministry said Sunday. It said Timoshenkov was protecting an area where the Russian military was handing out humanitarian aid to residents in Homs province on Wednesday and opened fire to stop a vehicle speeding up to the site. The car, rigged with explosives for a suicide attack, blew up, wounding the serviceman. He died the next day in a hospital.
Elsewhere, a suicide blast targeted the entrance to a park in the Syrian city of Qamishli, which is shared between government and Kurdish forces, killing three people, an hour after a memorial was held there for victims of the 1915 Armenian genocide.
"Had the bombing taken place earlier, there would have been a massacre," Qamishli resident and writer Suleiman Youssef told The Associated Press by telephone. Qamishli lies on Syria's northern border with Turkey.