Fighting in Sudan has intensified as warring factions seek to secure strategic locations, as pressure grows from international powers to end hostilities and allow humanitarian assistance to reach millions of desperate civilians. Fierce battles on Thursday between the Sudanese army and its paramilitary opponents, the Rapid Support Forces (RSF), reminded residents in Khartoum, the capital, of the fierce combat that marked the first days of the war almost three weeks ago. The clashes were particularly intense around the presidential palace at the centre of Khartoum, whose possession grants its occupants an appearance of legitimacy as rulers of Sudan. The sprawling complex is now badly damaged and reported to be largely held by the RSF. The continuing failure of combatants in Khartoum and elsewhere across Sudan to respect successive ceasefires prompted the US president, Joe Biden, on Thursday to threaten new sanctions against those responsible for “threatening the peace, security, and stability of Sudan; undermining Sudan’s democratic transition; using violence against civilians; or committing serious human rights abuses”. “The violence taking place in Sudan is a tragedy – and it is a betrayal of the Sudanese people’s clear demand for civilian government and a transition to democracy. It must end,” Biden said. Analysts and campaigners said the move was long overdue. More than 550 people have been killed in 20 days of violence, according to ministry of health statistics, but the true total is likely to be much higher as many deaths go uncounted. Thousands have been wounded. At least 334,000 people have been displaced inside Sudan and tens of thousands more to Egypt, Chad, South Sudan, Central African Republic and Ethiopia, according to UN agencies. The fighting has pitted forces loyal to Gen Abdel Fattah al-Burhan, the de facto military leader of Sudan, against those of the RSF’s commander, Gen Mohamed Hamdan Dagalo, who is also known as Hemedti. Both men now appear convinced they can win the current conflict and so gain unchallenged control over Sudan’s crumbling state and valuable resources. Anette Hoffmann, a research fellow with the Clingendael Institute, The Hague, said the intensity of recent battles was determined by a “political agenda … linked to the push for negotiations”. “I don’t see it likely for them to stop fighting unless one suffers really huge losses and can’t find the resources anywhere to replenish their forces,” Hoffmann told the Guardian. In Khartoum, residents sought shelter from bombardments and airstrikes for the 20th day. “Since yesterday evening, and this morning, there are … the sounds of clashes,” said Al-Sadiq Ahmed, a 49-year-old engineer in the capital. “We’ve got into a state of permanent terror because the battles are around the centres of residential neighbourhoods. We don’t know when this nightmare and the fear will end.” Experts say a humanitarian catastrophe is imminent, with aid agencies facing massive challenges in reaching the needy. “Our staff are extremely busy trying to coordinate between the various actors in order to facilitate humanitarian aid. But today in Khartoum, the absence of minimum security conditions prevents humanitarian workers from reaching vulnerable people. There have been reports of direct threats and attacks against humanitarian personnel including the Sudanese Red Crescent,” said Alyona Synenko, regional spokesperson for Africa at the International Committee of the Red Cross. “International humanitarian law is very clear on this: humanitarian organisations, its personnel and all resources and objects used for humanitarian operations must be respected and protected.” A statement posted by Sudan’s main maternity hospital said it had been overrun by the RSF and accused the force of stealing money. The RSF did not respond to a request for comment on the charge. Nabil Abdallah, the army’s spokesperson, accused the RSF of breaking the current ceasefire. “There was an announced ceasefire but they broke it by attacking our forces …. but now the situation is calm and there is no danger on the situation,” Abdallah told the Guardian. Earlier this week, Dagalo and Burhan agreed to send representatives for negotiations, potentially in Saudi Arabia. Such talks would initially focus on establishing a “stable and reliable” ceasefire, UN officials said, though warned against false hopes. Dagalo said the paramilitary has named its representatives to the talks but that trust-building measures had to be in place first. “A settlement should come after other matters: first, a ceasefire and building the trust,” he told Asharq, a Saudi-based TV station. An envoy of Sudan’s military leader said on Wednesday that the army “accepted the Saudi-American initiative for truce talks, not mediation to end the fighting” but ruled out face-to-face discussions with the RSF. Even indirect talks, if they take place, would be significant progress since fighting erupted on 15 April. The US and Saudi Arabia have led the international push to get the generals to stop fighting, then engage in deeper negotiations to resolve the crisis. The effort has been complicated by conflicting agendas of regional powers, and the involvement of dozens of smaller actors seeking to exploit the chaos. One possibility raised by the UN was to establish a monitoring mechanism that includes Sudanese and foreign observers. Talks on a sustained ceasefire could take place in either Saudi Arabia or South Sudan, which is favoured by regional powers and the African Union, officials said, though they pointed out that in either case combatants would need safe passage through the other’s territory. “That is very difficult in a situation where there is a lack of trust,” said Volker Perthes, the UN secretary general’s represenative in Sudan.