The United Nations has significantly revised the estimated death toll from Yemen's 18-month civil war to up to 10,000 people - a huge increase on the previous estimate of more than 6,000.

Speaking from the capital Sanaa on Tuesday, Jamie McGoldrick, the UN humanitarian coordinator, said the new figure was based on official information from medical facilities in Yemen.

The number could rise further, McGoldrick said, as some areas had no medical facilities, and people were often buried without any official record being made.

"We know the numbers are much higher but we can't tell you by how much," McGoldrick told reporters

"The figures we have are probably incomplete because we take the numbers from functioning health services, and in some of these areas there are no functioning health services," he added.

"People get killed or die and are actually buried before they are recorded, and we don't have a way of recording that."

Previous estimates cited by officials and aid groups for much of 2016 said the conflict had killed some 6,500 people, half of them civilians.

Worsening humanitarian conditions

Long-running negotiations have failed to end the conflict between government forces, backed by an Arab coalition, and Houthi rebels, who are supported by Iran.

Yemen descended into chaos after the 2012 removal of long-time President Ali Abdullah Saleh, whose forces are now fighting alongside Houthis.

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A Saudi-led Arab military coalition started air strikes against Houthis in support of the government of President Abd-Rabbu Mansour Hadi in March 2015.

Security deteriorated further after the Houthis swept into Sanaa in September 2014 and pushed south, forcing Hadi's government to flee into exile.

The conflict has displaced three million Yemenis and forced 200,000 people to seek refuge abroad, McGoldrick said.

The UN had information that 900,000 of those displaced intended to try to return to their homes, he said.

"This is a big challenge, especially in areas still experiencing conflict," McGoldrick said.

Some 14 million of Yemen's 26 million population need food aid and seven million are suffering from food insecurity, he said.