The UN’s special envoy to Yemen has expressed confidence that a deal can be struck between Houthi forces and the Saudi-backed Yemeni government to prevent an escalation of violence around the strategic port of Hodeidah.

The rebel-held Red Sea port – the distribution point for more than 70% of the aid reaching Yemen – is under assault from an array of forces, including those of Saudi Arabia and the United Arab Emirates.

Martin Griffiths is negotiating a ceasefire in which Hodeidah port would be handed over to full UN control and its revenues channelled to the Yemeni state bank. He held talks with the Houthi leadership in the Yemeni capital, Sana’a, and said he was “encouraged by the constructive engagement”.

In a statement he said he expected to meet shortly with President Abd-Rabbu Mansour Hadi, the leader of the UN-recognised Yemeni government. Hadi has been in exile and heavily dependent on Saudi sponsorship.

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The Houthi terms for handing the port to UN control have not yet met the conditions set by the Saudi-led coalition, and Griffiths warned “further military escalation would have severe political and humanitarian consequences”.

The UAE’s foreign minister, Anwar Gargash, has demanded an unconditional withdrawal of all Houthi forces from Hodeidah and claimed Houthi forces were seeking to “manufacture a humanitarian crisis” by blocking aid from being unloaded at the port and destroying supplies of water in the city’s residential areas.

But Amnesty International has accused coalition forces of deliberately delaying unloading of aid from the port over months, adding that its actions amounted to collective punishment and a war crime.

Amnesty said in a new 22-page report, entitled Stranglehold, that since 2015 the coalition had repeatedly tightened its blockade of the two Houthi-controlled ports of Saleef and Hodeidah, seriously impeding Yemenis’ access to food.

Using the cover of enforcing a UN arms embargo, the Saudis were slowing the docking of ships by carrying out inspections that delayed vessels by an average of 120 hours in March and 74 hours in April this year, the report said. The inspections were duplicating UN inspections of ships that had been introduced in 2015 after Saudi protests that Houthis were using aid convoys to smuggle Iranian arms.

The Saudis, under a deal brokered with the UN, are permitted additional inspections to those carried out by the UN, and if necessary can divert boats to a different port for full inspection.

Amnesty had tracked one shipment that was delayed for 50 days after it was cleared for unloading by the UN. A second was delayed for 46 days and a third for 24 days. Only one of the delays had been notified to the UN security council by the Saudi coalition as required under the UN agreement, Amnesty said.

Lynn Maalouf, Amnesty’s Middle East director, said: “The time that these inspections are taking is effectively obstructing the flow of humanitarian aid and essential goods, and that is why we have found that this could amount to collective punishment blockades that cause substantial, disproportionate harm to civilians are prohibited under international law.”

Amnesty also criticised Houthi distribution of aid, including the delays it imposed on aid workers seeking to distribute food out of Sana’a warehouses. Houthis were also seeking to direct the groups to which aid was distributed and demanding bribes to release food from warehouses.

In a sign that the crisis could escalate, despite the UN envoy’s optimism, Gargash said: “The UN, international aid groups and the media report Houthi militia are purposefully and deliberately seeking to manufacture a humanitarian crisis and exacerbate conditions in the port and city.

“The Houthis are blocking offloading of aid at Hodeida port; destroying water and sewage supplies.” He claimed Houthis were planting improvised explosive devices and other explosives in the city.

He said it was increasingly urgent that the Houthis leave the city unconditionally.

Reporters in Hodeidah said the Houthis were digging defences while civilians were largely staying in their homes. A collapse in water supplies in Yemen’s hot summer would be a disaster, aid groups said.

The UAE is largely overseeing the offensive on Hodeidah with a 20,000-strong force consisting mostly of Yemenis gathered from southern separatists, Red Sea coastal plain fighters and followers of a nephew of the late president Ali Abdullah Saleh.

These forces have captured western coastal towns to form a narrow strip of control from the UAE’s southern bases to the port. The coalition has captured the airport, but will need to enter more densely inhabited areas either to take the port or to cut off the main road out of Hodeidah.

