British nationals trapped in Sudan have been warned by the government that there is “no guarantee” of getting them out of the increasingly violent country after the ceasefire ends and they should leave immediately.

More than 530 people have been evacuated by plane from Sudan, but a three-day ceasefire is due to end on Thursday night, putting any future rescue flights in jeopardy.

The foreign secretary, James Cleverly, said evacuations had been made easier by the ceasefire, but it was impossible to predict what would happen when it ends.

“We cannot predict exactly what will happen when that ceasefire ends, but what we do know is it will be much, much harder, potentially impossible,” he told Sky News.

“So, what we’re saying to British nationals is: if you’re hesitant, if you’re weighing up your options, our strong, strong advice is to go through Wadi Saeedna [airstrip] whilst the ceasefire is up and running.

“There are planes, there is capacity, we will lift you out. I’m not able to make those same assurances once a ceasefire has ended.”

The government has faced criticism by some British nationals about the lack of communication and planning as they attempt to leave the country.

Speaking on BBC Radio 4’s Today programme, Cleverly said: “I understand how frustrating it must be when you are not able to get communication or get updates of what’s happened.”

He said the mobile phone coverage in Khartoum was inconsistent and UK representatives had been broadcasting messages through social media channels and WhatsApp groups.

Asked why other countries, such as Germany, China and the US, had evacuated more people, Cleverly said UK nationals were more dispersed than other expat communities.

But he refused to discuss the possibility of safe and legal routes for non-UK nationals fleeing Sudan to get to the UK.

“There is war and conflict all over the world. There are literally millions upon millions of people who are in countries plagued by war. We recognise that we cannot host everybody who is in a country plagued by war,” he said.

Asked about the home secretary’s comments that people arriving in the UK in small boats had values which were “at odds with our country”, Cleverly said: “We have always been a generous nation, but we can also see that there are people who are proactively abusing our hospitality.”

He added: “People are generous of spirit, but if we see people abusing that generosity, it erodes trust, it erodes trust in the immigration system.”

Cleverly has said he did not recognise reports that the UK had delayed Germany’s efforts to evacuate its citizens from Sudan at the weekend. The BBC said it had been told by senior German political sources that Britain’s mission to extract its embassy staff had led to a temporary blockade of the airfield near the capital, Khartoum, from which subsequent evacuation flights have taken off.

“I don’t recognise that reporting, that is not what has been communicated to us,” he said.

Speaking on the Today programme he said: “The reason we extracted our diplomats is because where the British diplomatic staff were located was right in the epicentre of the conflict and we know diplomatic staff were directly being targeted.”