Pro-Russian rebels pressed Friday to seize a key airport in eastern Ukraine despite fierce resistance by government forces.

An AP reporter on Friday saw three rebel tanks firing their cannons at the main terminal of Donetsk airport, where government forces have holed up. Sniper shots rang around the area.

Rebels have made some gains in the area near the airport, seizing some buildings on its fringes and using them to target the main terminal.

Ukraine's National Security and Defense Council spokesman Col. Andriy Lysenko said two Ukrainian servicemen have been killed and another nine wounded since Thursday. He said that Ukrainian forces at the airport have undergone rotation and firmly stood their ground.

The airport, located just north of Donetsk, the largest city in the east, gives the Ukrainian forces a convenient vantage point to target rebel positions. Its loss would be a major blow to Ukraine and would also allow the rebels to receive large cargo planes with supplies in addition to truck convoys from Russia.

Fighting for the airport has intensified this week, threatening to derail the truce declared Sept. 5. A follow-up deal which called for both parties to pull back their artillery to create a buffer zone hasn't been implemented.

Residential areas in Donetsk have been caught in the crossfire. A Red Cross staffer died Thursday when a shell landed near the group's office in Donetsk.

The rebels said the shelling came from the Ukrainian side, while Ukrainian Foreign Minister Pavlo Klimkin blamed the death of the Red Cross worker on "terrorists."

A spokesman for U.N. Secretary-General Ban Ki-moon issued a statement late Thursday, saying the aid worker's death, along with the shelling of a school that killed three people earlier this week, "underscore the fragility of the current cease-fire and the importance of ensuring a secure environment in south-eastern Ukraine that will allow humanitarian actors to carry out their work and deliver critical assistance to those most in need."