The situation in El Fasher, North Darfur, has become a humanitarian catastrophe. As fighting between the Sudanese Armed Forces (SAF) and the paramilitary Rapid Support Forces (RSF) escalates, more than 1.5 million people, including those in the Zamzam internal displacement camp, are at immediate risk. The threat of crimes against humanity is real and urgent. Civilians in the region are facing dire conditions, with mass violence, displacement and a paucity of essential resources.

On the other side of the world, in New York, the high-level week of the 79th United Nations general assembly is beginning, gathering the world’s leaders to address the most pressing international challenges. While the Middle East and Ukraine are set to dominate the agenda, we must not let the crisis in Sudan be ignored. The meeting is a pivotal opportunity – indeed, a responsibility – for collective action to ensure that the international community lives up to its obligation under the Responsibility to Protect (R2P) framework to help protect populations from destruction. Failure to do so risks the lives of more than a million people.

I first visited El Fasher in 2006, together with my late colleague, member of parliament Jo Cox. I spent the following years speaking out against atrocities committed in Darfur, atrocities so heinous that the former US president George W Bush described them as a genocide. Earlier this year, when I was the deputy foreign secretary, I found myself once again meeting refugees fleeing the current conflict as they poured over the border of Sudan into Chad. It was heartbreaking to witness, shameful that we are here again, and I was determined the UK should act.

The UK secured a security council resolution to “help secure a localised ceasefire around El Fasher and create the wider conditions to support de-escalation across the country and, ultimately, save lives”. The resolution also requested that António Guterres, the UN secretary-general, make further recommendations for the protection of civilians in Sudan. This report is being drafted, but the very catastrophe these protection options were intended to avert may well be under way before it is complete.

I believe these efforts earlier in the summer helped to buy more time and is one of the reasons the catastrophe we fear has not yet happened; diplomatic leadership can help protect civilians. Another factor was the rains. The rain season came and made fighting more difficult for several months. But now the weather is becoming drier and the time that we had has run out.

Lord Collins of Highbury, the new minister for Africa, committed earlier this month to ensuring that the options being drafted by the secretary general are implemented. But nothing happens swiftly or easily in New York and it will take persistent leadership and clear-eyed focus on the specific issue of protecting civilians to follow through on this promise.

I urge our new ministers who will be in New York this week to impress upon our allies the need for us to show steadfast resolve to find solutions – and at pace. They should also request that the security council receives regular briefings on the latest satellite imagery and analysis, because that is often where we can see most clearly the belligerents on the move. Our ministers must also communicate their expectations to the secretary-general that when he reports back to the security council on protection recommendations he supports the call for the deployment of an independent and impartial force with a mandate to protect civilians in Sudan.

This protection of civilians report by the secretary-general needs to be made public and then its recommendations must be both implemented and monitored by the security council.

As lead member, or penholder, on Sudan and the protection of civilians, special responsibility will fall to the UK to make sure this is done. Yes, there are urgent competing priorities in the Middle East and in Ukraine, but new ministers must be in no doubt of the severity of the situation facing El Fasher and the regional implications if it falls.

If we do not address the crisis head on, the world risks standing by as countless innocent lives are lost. The calls from human rights organisations must be amplified at the general assembly. Decisive international measures, including diplomatic pressure and humanitarian intervention, should be on the table.

I was one of the first ministers to call out the violence by the RSF in Darfur for what it is: ethnic cleansing. This reality cannot be disputed. Alice Wairimu Nderitu, under secretary-general and special adviser to the secretary-general on the prevention of genocide, has warned that “the situation today bears all the marks of risk of genocide, with strong allegations that this crime has already been committed”.

This is a critical time for global leadership to live up to our commitments to protect vulnerable populations from destruction. We are watching the Janjaweed, the murderous militia now restyled as the RSF, trying to finish what it began 20 years ago. We must not allow it to succeed.