Yemen's Iran-backed Houthi rebels have demanded payments for civil servants in areas under their control as a condition for renewing a UN-brokered truce in the country.

The current ceasefire is set to expire on October 1 and UN special envoy to Yemen Hans Grundberg has held talks with Saudi, Omani and Yemeni officials in an effort to extend it.

He met Mahdi Al Mashat, chief of the Houthi Supreme Political Council, on Wednesday in the capital Sanaa for talks on the truce.

Mr Grundberg was told the rebels would "not accept the expansion" unless the salaries of all state employees and the pensions of retired state staff were paid, the Houthi-run Saba news outlet reported.

"Our acceptance of the armistice without paying the salaries of state employees and the pensions of retirees is an acceptance of the continuation of the war and the siege on our dear Yemeni people in a way fiercer than the military war," Saba quoted the Houthi leader as saying.

While Yemen's internationally recognised government and the Houthis have, in principal, agreed on the need to pay civil servants, the sides are in dispute over the payroll.

The government said it was willing to abide by the payrolls before September 2014, when the Houthis seized Sanaa and began to hire thousands of people to work in de facto "government institutions".

The Houthis want payments to be made according to more recent records.

The truce came into effect in April has twice been renewed in two-month increments. Mr Grundberg hopes the warring parties will agree on a longer extension this time.

The truce has given Yemeni civilians a reprieve from the hardships of the conflict, with the amount of oil flowing through the port of Hodeidah increasing by four times.

About 21,000 people have been able to travel in and out of the country through the airport in Sanaa, UN figures show.

Civilian casualties have dropped by 60 per cent since April, the UN said.