KIEV - Three Ukrainian civilians died overnight in mortar fire around Donetsk airport, where government forces held out against pro-Russian rebels in the latest deadly violation of a ceasefire, officials said Friday.

The Donetsk mayor’s office said the clashes, in which five people were wounded, lasted nine hours. A Ukrainian military spokesman said government troops remained in control of the air hub, which was briefly seized by rebels in May, then recaptured. A truce in the six-month conflict signed by the warring sides and Moscow on September 5 has calmed some of the tensions in the Russian-speaking eastern rust belt. But the militias continue to mount daily raids on the international airport, which was once the busiest in eastern Ukraine and has a runway that could enable the separatists to receive essential supplies from Russia.

The uprising in the east, sparked by the February ouster in Kiev of a Russian-backed administration, has claimed some 3,400 lives and raised security concerns across eastern Europe. The Kremlin denies masterminding the conflict in reprisal for the new Ukrainian leaders’ decision to pull the ex-Soviet country out of Russia’s orbit and align it with the West.

But NATO has reported the movement of Russian tanks and special forces into the war zone and Kiev claims to have arrested Kremlin agents in the region. The United Nations’ human rights office said that continuing clashes have killed more than 300 people since the ceasefire agreement, although the Ukrainian government has only reported 115 deaths.