The EU-Turkey agreement to tackle the migrant crisis took effect on Monday but Greek authorities have warned the operation is threatened by a shortage of personnel.
A Downing Street spokesman said Mr Cameron spoke to Mr Tsipras on the phone on Tuesday afternoon.
The Prime Minister welcomed the start the Greek authorities had made to returning migrants to Turkey under the agreement, and reiterated the importance of breaking the link between people making the dangerous crossing of the Mediterranean and securing settlement in Europe... The Prime Minister confirmed that additional UK personnel would be offered to Greece over the coming weeks to provide additional support.
The Pope is to visit Greece, including the island of Lesbos, the Government there has confirmed.
Pope Francis and the Greek Orthodox Patriarch will visit the area on April 14 and 15, ITV News producer Maria Kagkelidou said.
Greece has postponed the return of the next group of migrants to Turkey until Friday, according to a Turkish official.
Greece began to return migrants back to Turkey on Monday, as part of an EU agreement to control the mass influx of people in Europe.
Thousands of people awaiting deportation to Turkey have applied for asylum in a bid to stay in Greece.
Thousands of people held in migrant detention camps as they await deportation to Turkey have applied for asylum in a bid to stay in Greece, authorities claimed today.
According to comments made by Maria Stavropoulou, director of Greece's Asylum Service, on state TV the influx of asylum applications 'could slow the rate of returns under the EU-Turkey deal'.
She said that some 3,000 people held in deportation camps on the islands are seeking asylum, with the lengthy application process to formally start by the end of the week.
Hundreds of migrants have been protesting in Lesbos today as they wait at a detention camp before being deported to Turkey.
ITV News' team in Lesbos caught them on camera as they chanted the word "freedom" over and over as they staged a sit-in at the Moira detention camp.
Turkey is set to take in an estimated 200 'irregular' migrants from the Greek island on Wednesday, a government official has confirmed.
It comes as an EU agreement for Turkey to take back migrants and refugees who cross the Aegean to enter Greece illegally came into effect yesterday.
Several hundred migrants have been pictured staging a sit down protest at a detention camp on the island of Lesbos.
Europe Editor James Mates reports that protesters shouted "I want freedom".
More migrants have been pictured arriving on the island of Lesbos, as Greece enters the second day of compulsory migrant repatriations.
Greece began to return migrants back to Turkey yesterday as part of an EU agreement to control the mass influx of people in Europe but the threat of deportation seems to have left new arrivals undeterred.
For the first time, after more than 1 million migrants and refugees entered Europe over the last year, compulsory repatriations began.
Each returnee was accompanied by their own guard from the EU border agency Frontex and for the 202 that returned to Turkey today, the dream of a better life in Europe was at an end.
The EU wants to put the networks of people smugglers out of business but the key test is not whether they can send enough people back but whether it deters people from coming in the first place.
ITV News Europe Editor James Mates reports.
Turkish President Tayyip Erdogan criticised European countries for not letting migrants into their territories by raising razor wire fences.
Speaking to the general assembly for humanitarian organisation Red Crescent in Ankara, he said that European countries "turned back Syrians" while the Turkish government had rescued 100,000 migrants from the Aegean Sea.
Did we turn Syrians back? No, we didn't. But they (European countries) did.
By way of placing razor wire, they didn't let these people in to their countries.
We see who's dying on the Aegean Sea. But the number of those rescued by us on the Aegean Sea is 100,000.
Greece has begun returning migrants to Turkey, home to 2.7 million migrants, as part of an EU agreement for financial and political concessions.