SANAA -- The UN envoy to Yemen Martin Griffiths arrived in the rebel-held capital Sanaa on Saturday, in an apparent bid to cement the fragile cease-fire in Yemen's port city of Hodeidah.

It was Griffiths' second visit to Sanaa in a month as the UN has been pushing for the peace process in Yemen to end nearly four years of devastating war, which has pushed the Arab nation of more than 20 million people to the brink of famine.

"I'm Sick and I Need to Travel Abroad for Treatment," read a banner raised by a group of sick children diagnosed with cancer and leukemia who were gathering at the Sanaa international airport.

"Yemen's Children are Suffering ... Lift the Blockade on Sanaa International Airport," said another banner.

An official with Houthi-run local media outlets told Xinhua on condition of anonymity that the UN envoy is expected to meet Abdulmalik al-Houthi, Houthi top leader, "to discuss the situation of Hodeidah and break the stalemate in implementing the cease-fire agreement."

Griffiths is also due to meet with President Abd-Rabbu Mansour Hadi and his Yemeni government in the Saudi capital of Riyadh, according to a UN statement released on Friday.

Earlier in the day, heavy clashes erupted between Yemeni government troops and Houthi rebels near Hodeidah University and Rabasah quarter at the southern edge of the port city.

Heavy machine guns and artillery shelling were heard across the city in the hours-long fighting, Mohammed al-Zabidy, a local resident, told Xinhua by phone.

The warring forces also traded accusations, blaming each other for breaching the truce and undermining the ongoing peace process.

Last week, the Houthi group said it began withdrawing from the Hodeidah ports in order to "implement the first phase of redeployment."

In response, the UN said the redeployment "would only be credible if all parties and the United Nations are able to observe and verify that it is in line with the Stockholm agreement."

Patrick Cammaert, appointed by the UN as head of the Redeployment Coordination Committee (RCC) which includes representatives from both Yemeni rival forces, would arrange the redeployment plans and mechanism required to monitor the cease-fire and ensure that "credible redeployment is achieved," it added.

Cammaert arrived in Hodeidah last week to oversee the implementation of the cease-fire between the Yemeni parties.

Under the truce, the withdrawal from the ports of Hodeidah, Salif and Ras Issa, and critical parts of the city associated with humanitarian facilities should be completed within two weeks after the ceasefire enters into force, while the full withdrawal should be completed within a maximum period of 21 days.

Saudi Arabia has been leading an Arab military coalition in Yemen against the Iran-backed Houthi militia since March 2015, in order to reinstate the exiled Yemeni government of Hadi.