LONDON: Britain said Thursday that it had moved 897 nationals out of Sudan as Foreign Secretary James Cleverly urged those remaining to use available evacuation flights before the end of a temporary ceasefire.

London has laid on eight flights, including two on Thursday, since launching civilian airlifts late Tuesday, according to the Foreign Office.

Doubts are growing about the ability to continue the operation, after heavy battles occurred on the second full day of a three-day US-brokered truce.

"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," Cleverly told Sky News of continuing the evacuations beyond Thursday.

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"So what we're saying to British nationals is if you're hesitant, if you're weighing up your options, our strong advice is to go.

"Whilst the ceasefire is up and running, there are planes, there's capacity, we will lift you out. We are not able to make those same assurances once a ceasefire is ended."

The UK has been using an airstrip near the capital Khartoum to fly its citizens, their dependants and other foreign nationals out of the conflict-mired country.

They are being taken to a British military base in Cyprus, before onward travel on to the UK.

London had faced domestic criticism for initially extracting only diplomats and their families at the weekend, and for lagging in the number of people it had airlifted out.

Cleverly defended the government's approach in a round of broadcast interviews, arguing "different countries are operating evacuations in different ways".

For British nationals, "the pattern that we've seen is they tend to be more distributed around the city, often have Sudanese nationals as part of their families (and) this makes the evacuation much more complicated," he told BBC Radio.

More than 2,000 British nationals have registered under the evacuation plans.