SANAA, Feb. 17 (Xinhua) -- UN Special Envoy to Yemen Martin Griffiths arrived on Sunday in the rebel-held capital Sanaa to follow up the implementation of Stockholm Agreement on Hodeidah.

Griffiths headed to downtown Sanaa to meet Abdulmalik al-Houthi, leader of Houthi group, the rebel-run al-Masirah TV reported.

This was Griffiths' fifth trip to Sanaa in two months in his attempts to break a stalemate in the implementation of a cease-fire to withdraw warring forces and secure access to grain aid in the country's Red Sea port city of Hodeidah.

Griffiths has been shuttling between the Houthi rebels in Sanaa and the exiled government in the Saudi capital of Riyadh to avert an all-out deadly fighting in Hodeidah, the lifeline of the country's most commercial imports and humanitarian aid.

In his last week's visit to Sanaa, Griffiths warned that about 51,000 metric tons of grain aid stored in mills in the besieged Hodeidah city was "at risk of rotting," asking the rival parties to allow the UN team for urgently access to the mills to deliver the aid to the extremely needy people.

He said that the grain aid is enough to feed over 3 million people, but the World Food Programme (WFP) has been inaccessible for over five months to reach it.

On Thursday, Stephen Anderson, the WFP representative and country director in Yemen, told Xinhua in an interview that he expected the warring parties to reach an agreement soon to facilitate access by humanitarian workers to the mills.

"There are 20 million people in Yemen who are considered hungry now, which is 70 percent of the population ... We are trying to assist the most needy people," Anderson said.

The four-year civil war has pushed over 12 million people to the verge of famine and created what the UN calls the world's worst humanitarian crisis.

The warring parties reached a peace deal in Stockholm in December last year. They have largely held the cease-fire deal in Hodeidah but failed to withdraw their forces.

The Houthi rebels continue to fortify themselves inside the city while the government troops have been massing on the southern and eastern outskirts.

Michael Anker Lollesgaard, head of the UN cease-fire monitor team in Hodeidah, has resumed joint meetings between the representatives of the warring parties inside the city to discuss the forces' withdrawal.

Pro-government media outlets reported that the UN was trying to convince the rival parties to allow an international peacekeeping force into Hodeidah to secure the city after the withdrawal of the local warring forces.

However, there were no comments by the UN over those reports.