Trump on Istanbul attack: 'Will the world ever realize what is going on?'

Presumptive Democratic presidential nominee Hillary Clinton has said the terror attack in Turkey is a reminder that the US can't retreat from this part of the world and needs to deepen co-operation with allies in the Middle East and Europe to take on this threat.

"Today's attack in Istanbul only strengthens our resolve to defeat the forces of terrorism and radical jihadism around the world. And it reminds us that the United States cannot retreat," Clinton said in a statement on Wednesday.

"We must deepen our cooperation with our allies and partners in the Middle East and Europe to take on this threat," she said adding that such cooperation is essential to protecting homeland and keeping the country safe.

"Yet another terrorist attack, this time in Turkey. Will the world ever realize what is going on? So sad," Republican presumptive presidential nominee Donald Trump tweeted.

"We must do everything possible to keep this horrible terrorism outside the United States," Trump said.

In a statement, the Trump campaign said the terrorist threat has never been greater.

"Our enemies are brutal and ruthless and will do anything to murder those who do not bend to their will. We must take steps now to protect America from terrorists, and do everything in our power to improve our security to keep America safe," the Trump campaign said.

Former Republican presidential candidate and Florida Senator Marco Rubio said the US stands with ally Turkey in condemning this attack.

"We stand ready to assist them as we learn more about the perpetrators responsible for this," he said.

Meanwhile, the US and UN has called for stepped-up cooperation in fighting terrorism and has vowed "steadfast" support for Turkey.

"Ataturk International Airport, like Brussels Airport which was attacked earlier this year, is a symbol of international connections and the ties that bind us together," the White House Press Secretary Josh Earnest said on Wednesday.

US President Barack Obama was briefed on the explosions by his Assistant for Homeland Security and Counterterrorism Lisa Monaco.

"Our deepest condolences go out to the families and loved ones of those killed, and we wish a speedy recovery to those injured. We remain steadfast in our support for Turkey, our NATO Ally and partner, along with all of our friends and allies around the world, as we continue to confront the threat of terrorism," he said.

Condemning the terrorist attack at Istanbul's AtaturkInternationalAirport, the UN Secretary General Ban Ki-moon, expressed his deepest sympathy and condolences to the families of the victims and to the Government and people of Turkey.

"The Secretary-General hopes that the perpetrators of this crime will be identified and brought to justice. He stands firmly by Turkey as it confronts this threat and stresses the need to intensify regional and international efforts to combat terrorism and violent extremism," his spokesman said in a statement.