The humanitarian evacuation of 250 people from two Syrian towns besieged by pro-government forces and another 250 from two towns blocked off by rebels began on Wednesday, coordinated by the Syrian Arab Red Crescent.

The towns blockaded by pro-government forces are Zabadani and Madaya, near the Lebanese border. Those blockaded by rebels are al-Foua and Kefraya in the northwestern Idlib province, both predominantly Shiite and loyal to the government.

Some 500 people — including the sick, wounded and their family members — are being evacuated and some urgently need life-saving medical attention, a U.N. official said.

"Nine of them need transfer by ambulance, others are being bussed either to Damascus or Idlib," another U.N. official in Geneva told Reuters.

Jan Egeland, chairman of a U.N. humanitarian task force on Syria, said last week he was disappointed by a slowdown in aid access, largely due to blockages by government-backed forces.

But he voiced hope that the evacuation of 500 people from the four towns would happen this week.

A Syrian Arab Red Crescent car leaving Madaya and Zabadani came under rebel fire earlier on Wednesday but there were no casualties, a witness told Reuters.

The Lebanese TV station al Mayadeen also reported the Red Crescent convoy had come under rebel fire.

Residents of Madaya and Zabadani will be taken to the rebel-held Syrian town of Idlib, the witness said.

Coaches with Red Crescent markings lined up for inspection before heading to al-Foua and Kefraya, a Reuters witness said.

Pro-opposition TV station Orient News said residents from al-Foua and Kefraya would be taken to the government-controlled city of Latakia and to the Damascus area.

The warring sides agreed a local cease-fire for the four towns in September, but the agreement has not been fully implemented.