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British nationals in Cairo, Alexandria and Suez are being told to leave if it is safe for them to do so, following days of violent protests across Egypt.

But despite upgrading its advice, the Foreign Office (FCO) is not currently organising a formal evacuation.

Foreign Secretary William Hague said he was concerned by the number of Britons trying to leave at Cairo airport.

While flights were coming in and out, a lack of staff meant it was not functioning properly, he said.

Meanwhile PM David Cameron and US President Barack Obama called for an "orderly transition" of government.

After discussing the crisis on Sunday, the two leaders said the north African country needed a comprehensive process of political reform, a Downing Street spokeswoman said.

They urged the Egyptian government to respond peacefully to protests and condemned the violence of recent days.

Mr Cameron also spoke to King Abdullah of Jordan about the ongoing violence, amid fears that extremists could try to exploit the situation.

The British Ambassador to Egypt, Dominic Asquith, said there were "lots of challenges" at Cairo airport.

"That's why we've got the team up there trying to help," he sad. "There are flights going in and out but it is not orderly."

Several flights to Britain have been cancelled, people are unable to use the internet to book, and schedules have been affected by the curfew imposed in the city.

The curfew in Cairo, Alexandria and Suez was extended by an hour on Sunday night to run from 1500 to 0800 local time rather than 1600 to 0800, Egyptian state television reported.

Most of the 20,000 UK tourists in Egypt are in Red Sea resorts, which the FCO considers to be safe.

The UK Foreign Office to Cairo, Alexandria, Suez and Luxor, and say that anyone "without a pressing need" to be the first three of those cities should leave if it wass safe for them to do so.

Foreign Office minister Alistair Burt told the Â鶹ԼÅÄ: "We do want people to take the opportunity if they are able to leave... but as yet the situation has not reached the stage where we would necessarily be considering chartering planes and getting larger numbers out."

The FO says arrangements are being made for the spouses and children of British embassy staff in Cairo to leave the country on ordinary commercial flights.

Mr Hague said the welfare of British nationals was his top priority and he had sent extra consular staff to Cairo airport.

The foreign secretary also warned that Egypt could fall into the hands of extremists unless there were peaceful reforms.

He told the Â鶹ԼÅÄ: "It's to avert those risks and meet the legitimate grievances and aspirations of the Egyptian people that we are urging the Egyptian authorities... to create a more broadly-based government."

He said reforms should be "real and visible" and elections "free and fair".

The US embassy in Cairo is telling Americans to consider leaving the country as soon as possible and will begin evacuation flights on Monday.

In Cairo, demonstrators are back on the streets for a sixth day, demanding the resignation of President Hosni Mubarak.

Alex Belfield, a presenter with Â鶹ԼÅÄ Leeds who is on holiday in Sharm El Sheikh, said tourists at his hotel had been told they could not leave the building for at least 24 hours.

"This whole thing seemed a million miles away yesterday, but last night... when we got back [from dinner] the whole atmosphere had changed.

"The barricades had gone up at the front of the hotel... and there are 14 security guards in total, making it very clear we were not able to leave."

Sean Tipton, from travel body Abta, said UK tourists required to travel to Luxor, which is a popular start and end point for Nile cruises, were "being taken to the cruise very quickly and got out of the place very quickly".

The FO says it has had no reports of any trouble in Sharm El Sheikh.

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