UNITED NATIONS (Reuters) – Both sides in the conflict in Yemen have largely stuck to a ceasefire agreed last month, but substantial progress is still needed before more talks can be held on ending the war, the U.N. special representative to the country said on Wednesday.

Martin Griffiths told the United Nations Security Council he had met the leaders of the two sides in recent days and both had expressed determination to find a way forward.

“I am pleased to report that both sides have largely adhered to the ceasefire we agreed in Stockholm,” Griffiths said. “There has been a significant decrease in hostilities since then.”

He said while there had been some violence, it had been remarkably limited compared with in the lead-up to Stockholm.

However, while there was a sense of tangible hope and optimism, there was also concern, Griffiths said.

He said he and the leaders of both parties shared the view that “substantial progress, particularly on Hodeidah, is something we would like to see before we reconvene the next consultations.”

“I am still hopeful that we can proceed to a next round of consultations within the near future and I am working with both parties to make sure that that will happen at the earliest possible date,” he said.

At the end of peace talks in Sweden, the United Nations said another round of consultations would be held in January on a wider truce in the country, a framework for political negotiations and transitional governing body.

Griffiths said he had met the President of the Saudi-backed government Abd-Rabbu Mansour Hadi, who was ousted from the capital Sanaa in 2014, in Riyadh on Tuesday and with Houthi leader Abdul Malik al-Houthi, whose forces control most urban centers in Yemen including Sanaa and Hodeidah, on Sunday.

A major challenge lies in securing an orderly troop withdrawal from Hodeidah, the main port used to feed Yemen’s 30 million people that has been the focus of fighting over the past year.

Griffiths said the United Nations was working with both parties to finalize a list of prisoners to be exchanged as part of a prisoner swap agreed in December and he hoped a meeting of the supervisory committee for this could be held in Amman next Monday.

He said work was continuing to try to secure support for the central bank and to reopen Sanaa airport before the next round of talks, both of which would significantly ease humanitarian suffering.

The central bank, split into two rival head offices, has been slow to finance imports of food needed to fend off widespread hunger and is struggling to pay public-sector wages as foreign exchange reserves dwindle.

Sanaa airport is in Houthi territory but access is restricted by the Saudi-led military coalition, which controls the air space. Hadi’s government wants international flights inspected before flying in or out of Sanaa, but the two sides did not reach agreement in Sweden on where that would happen.

The war and the ensuing economic collapse has left 15.9 million Yemenis facing severe hunger.

Western nations, some of which supply arms and intelligence to the Saudi-led coalition that intervened in the war in 2015 to restore Hadi’s government, have pressed for an end to the conflict that has killed tens of thousands of people and spawned an urgent humanitarian crisis.

U.N. aid chief Mark Lowcock told the Security Council that while the Stockholm agreement was having an impact, the humanitarian situation remained “catastrophic,” with millions of Yemenis hungrier, sicker and more vulnerable than a year ago.

He said that while the situation with fuel imports had improved, commercial food imports had plummeted in December and he called on the government and others to allow unimpeded flow of imports including humanitarian aid.