MANILA, Philippines -- A US helicopter was shot at while ferrying patients and doctors from a port in Cotabato City to a US Navy hospital ship on Monday, a regional military spokesman said.

There were no casualties.

There was no immediate assessment of the damage to the aircraft, which was able to land safely, Lieutenant Colonel Julieto Ando, spokesman of the Army's 6th Infantry Division, said in a phone interview on Tuesday.

The helicopter was airborne at the time of the attack around 3:30 p.m., Ando said, adding that the identity of the attackers have not been determined.

The USNS Mercy is docked one kilometer off the Polloc port in Cotabato City. Patients and doctors are ferried to the ship by boat and helicopter, Ando said.

The US Navy ship is on its second visit to the Philippines in as many years. The mission hopes to treat an estimated 15,000 patients and help with construction projects.