DONETSK, Ukraine — Rebels in eastern Ukraine appeared Wednesday to be successfully closing in on the government-held airport in Donetsk, the seizure of which would be a strategic victory for the pro-Russian separatists.

At least 10 people were killed as residential areas near the airport were caught in the crossfire, further undermining a shaky truce that was imposed last month and has been riddled by violations since.

The rebels and government troops have been waging a particularly brutal fight for the airport. As the Ukrainian-held position closest to Donetsk, it has given government forces a convenient position to target the main rebel stronghold and a defeat there would be a major strategic and symbolic loss.

While it was impossible to get within close range of the airport because of the ongoing fighting, an AP reporter in Donetsk on Wednesday saw that artillery fire hitting the airport was coming from government-held positions outside the city — an indication that the airport may no longer be under Kiev's control.

Ukraine's National Security and Defense Council spokesman Andriy Lysenko told journalists in Kiev that the airport was still under control of government troops who were "brilliantly carrying out their duty" and holding ground there.

However, rebel leader Alexander Zakharchenko was quoted by Interfax news agency as saying that the rebels control 90 percent of the airport, which has been the focus of the worst fighting in the region for weeks.

"In two, or maximum three, days the Donetsk airport will come under our control," he said.

Civilian and military casualties have continued to rise despite a cease-fire imposed on Sept. 5. A second agreement between Ukraine, Russia, and the rebels was signed on Sept. 20 that required both sides to remove heavy artillery from the front-line to create a buffer zone between Ukraine and rebel positions.

While the second agreement helped enforce the cease-fire in certain parts of the region where Ukraine troops and rebel forces chose to retreat, other areas have been the scene of non-stop fighting since.

The increasingly violent battle for the airport has caught neighborhoods in the north of Donetsk in the crossfire, driving the civilian death toll even higher in a conflict that has claimed at least 3,500 lives since April.

The AP reporter saw bodies of three people killed after a shell exploded in a school courtyard in the north of the city. The city council of Donetsk said that in total four people had died, and that about 70 schoolchildren were in the school at the time of the attack but that those killed were adults.

Soon after the school was hit, another shell fell on a bus stop nearby. The AP saw two people who had been killed at the bus stop as well as another person on the crosswalk nearby. A minibus that was also hit was still burning hours later, though the AP was unable to confirm how many people were in the bus. The Donetsk city council said the total number of killed at the bus stop was six, and that several people were wounded.

In Moscow, Russian Foreign Minister Sergey Lavrov urged the West to look into the allegations of mass graves of civilians in eastern Ukraine.

"It's a terrible tragedy. It's an obvious war crime," he said at a briefing. "We hope that Western capitals will not keep silent about these outrageous facts."

Lavrov said that more than 400 bodies have been found in mass graves near Donetsk, but Andrei Purgin, a rebel leader in Donetsk, clarified Wednesday that the figure referred to the total number of unidentified bodies kept in Donetsk morgues.

Purgin said that the bodies of nine civilians have been found in a mass grave near Donetsk. Authorities in Kiev have rejected the rebel allegations that they had been killed by government forces.

Laura Mills in Kiev, Ukraine, and Jim Heintz in Moscow contributed reporting.