Houthi rebels in Yemen have threatened to launch more drone attacks after a deadly strike last week on a Yemeni government military parade killed seven people, stoking tension between the warring parties and threatening UN efforts to broker peace.

Houthi spokesman Yahya Sarea said Thursday’s drone strike on a military base in Lahaj province, which killed several people, was a “legitimate operation against aggression”. He said the movement was building a stockpile of locally manufactured drones.

“Soon there will be enough in the strategic stockpile to launch more than one drone operation in multiple battle fronts at the same time,” Sarea told reporters in the Houthi-held capital, Sana’a, on Sunday.

The Houthi statement came as Britain said it was pressing ahead in seeking an enhanced mandate for a UN mission to oversee a ceasefire in the port city region of Hodeidah, despite claims by the Saudi-backed Yemeni government that the Houthis were not implementing the agreements struck between the parties in talks in Stockholm last month.

The Yemeni intelligence chief, Brig Gen Saleh Tamah, died on Sunday from injuries sustained in the Houthi drone attack on Thursday. The attack was outside the ceasefire zone, but has been condemned by the US as a breach of the spirit of the Stockholm agreement.

In a further development, Houthi negotiators boycotted a meeting of the UN’s ceasefire monitoring body, the Redeployment Coordination Committee, on Sunday, saying the UN special envoy was using the committee to pursue other agendas.

Britain is nevertheless planning to table a fresh security council resolution next week to give a UN team in Hodeidah a fresh mandate after the current one expires on 21 January.

The new mandate will give the UN mission, overseen by the retired Dutch general Patrick Cammaert, enhanced powers to monitor the compliance of the parties to the ceasefire in Hodeidah governorate, and the mutual redeployment of forces from Hodeidah, and the ports of Salif and Ras Isa.

The mission will also discuss with both sides how security in the city of Hodeidah and the ports can be overseen by “a new local security force in compliance with Yemeni law”, a vague phrase in the Stockholm agreement that has so far been interpreted differently by the parties. The Yemeni government claims the agreement means its forces can start policing the city, something the Houthis – currently in control of the city – reject.

Hodeidah is the lifeline port for aid across Yemen.

The fresh resolution is also likely to result in an increase in the number of UN monitors capable of overseeing breaches of the ceasefire, with a total team of 75. The monitors may be drawn from other UN existing missions in Yemen or sent to the country for this specific purpose.

Both sides are accusing the other of repeated ceasefire breaches, and pressure is being applied on the US by Saudi Arabia and the United Arab Emirates to do more to condemn the Houthis.

Key issues remain access to humanitarian supplies, administration of the port of Hodeidah, the pullback of Houthi forces from the city and the composition of the new security force. Few diplomats would deny that the agreement is only being partially honoured, even if they would argue the levels of violence in the area have fallen.

The Aid group Oxfam is one of many that fear that the vagueness of the ceasefire could lead to its collapse. Dina El-Mamoun, Oxfam’s head of policy and advocacy in Sana’a, recently said: “There is an issue with the actual agreement, which is actually quite vague. The UN should have made clear these basic issues that go to the heart of the agreement: who needs to hand over what and to whom.”

A Foreign Office spokesperson said: “As penholder on Yemen in the UN security council, the UK is determined to support the important agreements made in Stockholm at the end of 2018. A new UN security council resolution to establish a UN mission monitoring the Hodeidah ceasefire will be another step forward. This is a man-made conflict and it will have man-made solutions. We are working hard in support of the UN special envoy and urge all parties to maintain their backing for the UN peace process.”

One difficulty is that the ceasefire is not nationwide, so the Houthi attack on Al Anad airbase in Lahij on Thursday using drones, was not technically in breach of the Stockholm agreement, but hardly helps enhance trust between the two sides.

The US state department condemned the drone attack, saying it breached the spirit of the ceasefire, and adding: “We urge all sides to honour the commitments they made in Sweden to their fellow Yemenis by refraining from violence and provocative acts.”

The US secretary of state, Mike Pompeo, is currently on a Gulf tour where he is assessing the willingness of the Saudis to give the Stockholm agreement further time to bed in.