Sanaa (AFP) - The Red Cross on Friday turned down a call from Riyadh for aid workers to stay out of rebel-held zones of Yemen that are being targeted in a Saudi-led air war.

The mission of the International Committee of the Red Cross "often necessitates crossing frontlines" and securing access to combat zones, the ICRC's Sanaa spokeswoman Rima Kamal told AFP.

"To reach all those in need in Yemen ... we will continue to seek security guarantees from all parties and in all regions," she said.

"We have no plans to change that for the time being, and we remain committed to operate in all regions and to do everything possible to reach the civilians affected."

According to a letter seen by AFP on Thursday, Saudi Arabia has asked the United Nations to move aid workers away from rebel-held areas as the Riyadh-led coalition presses on with air strikes in support of the Yemeni government.

The United Nations flatly rejected the request and reminded Saudi Arabia of its obligations to allow humanitarian access in Yemen, where coalition warplanes have been pounding Shiite Huthi rebels for nearly a year.

UN aid chief Stephen O'Brien said in a letter to Saudi Arabia's UN ambassador, Abdallah al-Mouallimi, that relief organisations were "delivering life-saving assistance as per internationally recognised principles and will continue to do so."

Responding to O'Brien, the ambassador said the request should not be "misinterpreted to indicate any hindrance to humanitarian access and the delivery of humanitarian assistance in Yemen".

The United Nations has had several disputes with Saudi Arabia over aid access in Yemen, where 80 percent of the population is facing dire food shortages.

O'Brien told Saudi Arabia that aid workers would continue to inform coalition authorities of their movements.

UN and international aid workers have passed on their coordinates to coalition military authorities to ensure they are not inadvertently targeted.

More than 6,100 people have been killed in the conflict since last March, about half of them civilians, according to UN estimates.