ADEN (Reuters) – Explosions rocked Yemen’s Red Sea city of Hodeidah for a second day on Wednesday despite a U.N.-mediated ceasefire meant to pave the way for peace negotiations to end nearly four-years of war.

Residents said six missiles blasts near the 7 July eastern suburb broke the calm, but it was not clear who was responsible.

The Iranian-aligned Houthi movement and the Saudi-led government had traded blame for violations on the first day of the truce on Tuesday, when residents reported shelling on the eastern and southern outskirts of the Houthi-held city at night.

The United Nations brokered the truce deal as part of confidence-building measures at peace talks last week in Sweden to avert a full-scale assault on the port that is vital for urgent aid supplies for millions facing starvation.

A source in the Saudi-led coalition arrayed against the Houthis told Reuters that if international monitors were not deployed in Hodeidah soon, the deal could falter.

“If the U.N. takes too long to get into (the) theater, they will lose the opportunity altogether and the Stockholm agreement will be a dead duck,” said the coalition source, who declined to be named.

A U.N.-chaired committee formed to oversee the truce and troop withdrawal from Hodeidah city and three ports held its first meeting on Wednesday using video link and phone with representatives from both sides.

Retired Dutch General Patrick Cammaert, chair of the Redeployment Coordination Committee, will travel to Jordan on Thursday, then on to Sanaa and Hodeidah, with a small initial advance team, U.N. spokesman Stephane Dujarric said.

The ceasefire deal, which covers only Hodeidah, will see international monitors deployed in the city and port with all armed forces pulling out within 21 days of the truce.

The two sides had also agreed a prisoner swap. A Red Cross official said in Geneva on Wednesday they had exchanged lists of a total of 16,000 people believed to be detained.

HODEIDAH A LIFELINE

Hodeidah, the main port used to feed Yemen’s 30 million people, has been the focus of fighting this year, raising fears abroad that a full-scale assault could cut off supplies to nearly 16 million people suffering from severe hunger.

The truce, the first significant breakthrough in peace efforts in five years, is meant to pave the way for a wider ceasefire in the impoverished country and a second round of talks in January on a framework for political negotiations.

A Sunni Muslim Arab alliance led by Saudi Arabia and the United Arab Emirates entered the war in 2015 against the Houthis to restore the internationally recognized government of Abd-Rabbu Mansour Hadi, which was ousted from the capital Sanaa.

Western nations, which supply arms and intelligence to the alliance, are calling for an to the conflict which has killed tens of thousands of people and is widely seen as a proxy war between Saudi Arabia and Iran.

The Security Council is considering a resolution to ask the U.N. chief to submit proposals by the end of the month on how to monitor the truce and forces redeployment.

The Houthis control most urban centers, including Sanaa, while Hadi’s government is based in the southern port of Aden.

Yemen descended into war after pro-democracy unrest forced late former president Ali Abdullah Saleh to step down. Hadi was elected to a two-year term to head a transitional government but the Houthis drove him into Saudi exile.

The Houthis deny getting any help from Iran and say they are waging a revolution against corruption.