9/11 anniversary: Barack Obama calls for unity against terrorism

The 15th anniversary of the horrific 9/11 attacks saw President Obama's call for unity against terrorism, Democratic presidential candidate Hillary Clinton's fainting and Republican presidential candidate Donald Trump's only statement on social media.

US President Barack Obama on Sunday led millions of Americans in paying moving tribute to the victims of the 9/11 attacks urging Americans to stand united against terrorism.

During a ceremony at 15th anniversary 9/11 attacks at the Pentagon he said, "together, there is nothing we Americans cannot overcome."

At the Pentagon, a trumpet played as the President took part in a wreath-laying ceremony.

"We remember, and we will never forget, the nearly 3,000 beautiful lives taken from us so cruelly, including 184 men, women and children here, the youngest just three years old. We honour the courage of those who put themselves in harm's way to save people they never knew," Obama said in a statement.

"We come together in prayer and in gratitude for the strength that has fortified us across these 15 years. And we renew the love and the faith that binds us together as one American family," he added.

"It is so important today that we reaffirm our character as a nation, of people drawn from every corner of the world, every colour, every religion, every background," he said.

"Bound by a creed as old as our founder ... Out of many, we are one. For we know that our diversity, our patchwork heritage, is not a weakness, it is still and always will be one of our greatest strengths," Obama said.

Obama also vowed that the US will never bow to the wishes of terrorists who, through their attacks, hope to change the way Americans live.

"We have delivered justice to Osama bin Laden. We have strengthened our homeland security. We have prevented attacks. We have saved lives. We resolve to continue doing everything in our power to protect this country that we love," he noted.

"...And today, we once again pay tribute to these patriots, both military and civilian, who serve in our name, including those far away from home in Afghanistan and Iraq," Obama said.

Meanwhile, Donald Trump released a statement on Facebook page and offered his condolences to the victims.

"Fifteen years ago, America suffered the worst terrorist attack in its history. Today, we mourn for all the lives lost. We mourn for all the children who had to grow up without a mom or dad, and for all the parents who have had to struggle on without their children," he wrote.

He praised the firefighters, police officers and port authority workers who rushed into danger, into smoke, into fire to save the lives of people they had never met, performing their sacred duties until their last moments on Earth.

"Today is a day of sadness and remembrance. It is also a day of resolve. Our solemn duty on behalf of all those who perished that September day 15 years ago, is to work together as one nation to keep all of our people safe from an enemy that seeks nothing less than to destroy our way of life," Trump noted.

Hillary Clinton, who had fainted at a public event due to "overheating" and was later diagnosed with pneumonia, said the attack was the closest thing to "hell" she had ever seen.

She visited the Ground Zero memorial to pay her respects and observe the annual moment of silence but soon fell ill.

According to the reports, Clinton took rest in her daughter's Manhattan apartment.

"She was put on antibiotics and advised to rest and modify her schedule. She is now re-hydrated and recovering nicely," her physician Lisa Bardack said.