US Vice President Joe Biden and his wife Jill signed a condolence book at the Turkish embassy in Washington for victims of the Istanbul airport attack.
Islamic State militants were suspected to have carried out Tuesday's attacks , which killed 42 people and wounded 239 at Europe's third-busiest airport.
He said the attacks on the airport had the effect of unifying the world against "distorted jihadists" but that defeating them would take some time.
I am absolutely convinced that combined with our resolve, we will wipe out the so called caliphate in Iraq and in Syria.
The world is united behind Turkey in our resolve.
"By killing his children, Mr Sheats will have to live the rest of his life with this horrible memory."Read the full story ›
This CCTV image, published by Turkish media, claims to show one of the alleged attackers at Istanbul's Ataturk Airport moments before the terror attack.
US counter terrorism officials believe the group involved in the attack at Istanbul's Ataturk Airport contains "dozens more members" according to ITV News' Security Editor Rohit Kachroo.
Three suicide bombers attacked the airport on Tuesday killing at least 43 people and injuring over 230.
The death toll from the attack on Istanbul's Ataturk Airport has increased to 43, Turkish officials have confirmed.
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Some of the 41 victims of the terror attack at Istanbul's Ataturk Airport are being buried as Turkey observes a national day of mourning.
A video has emerged showing people in Ataturk Airport in Istanbul running for their lives as three suicide bombers detonated bombs and exchanged gunfire with security forces.
The CCTV footage, which has no sound, shows people fleeing in panic just before a powerful explosion and the ceiling collapses covering everything in dust.
- Warning: Some viewers may find this footage distressing
People due to travel to Istanbul should speak to their travel company or airline if they have concerns about their journey, the Association of British Travel Agents (ABTA) has said.
It said travellers should follow Foreign Office advice, which says tourists should be alert to their surroundings.
An ABTA spokesman said: "People due to travel should speak to their travel company or airline for information about travel to and from Istanbul. Flights to and from Ataturk International Airport are resuming gradually."
Most British holidaymakers visit the beach resorts on Turkey's southern coast, the association said, adding that "Istanbul’s popularity as a city break destination has declined" following incidents earlier this year.
David Cameron has condemned the "horrific" attack at Istanbul airport.
The prime minister told the House of Commons that he had spoken to Turkish President Erdoğan and offered the UK's assistance.
"Our thoughts and prayers are with those who were killed and injured and their families," Mr Cameron said.
"As yet there are no reports of any UK causalities but the Foreign Office are working urgently with the Turkish authorities to establish the full facts.
"We stand as one in our defiance against these barbaric acts."