David and Samantha Cameron pay respects at Ground Zero
David Cameron and his wife Samantha have paid their respects to the 9/11 terror attack victims during a visit to New York City's Ground Zero.
Mrs Cameron, who had been in the city on the day of the attacks, laid a posy of white roses near the engraved name of British victim Katherine Wolf.
The prime minister said the memorial was "the place to remember why" action in Afghanistan had been necessary.
The visit came on the final day of an official state visit to the US.
Earlier he and US President Barack Obama discussed the possibility of releasing emergency oil reserves in an effort to dampen soaring prices.
The couple were shown around the memorial by New York mayor Michael Bloomberg before meeting families of British victims.
Nearly 3,000 people died when four hijacked airliners were crashed into the World Trade Center in New York, the Pentagon and a Pennsylvania field.
On Thursday both stood in silence close to the reflecting pool on the footprint of the former North Tower of the World Trade Center.
Mr Cameron told the BBC: "Here at the site of the Twin Towers, Ground Zero, here is the place to remember why what we do overseas is so important, so people are safe at home."
His wife was heard to say she would never forget the day it happened.
"I often think about the people who died and their families - I am glad I have been able to come and pay my respects properly today," she said.
They spoke to Charles Wolf whose Welsh wife Katherine was among those killed that day.
He said: "It was very, very nice for both he and his wife to come. It was very personal as well as being official.
"Of course they have had their own loss of their child, and when you are with someone who's had a recent loss like that, you know you are with people who get it, who care."
Afterwards Mr Cameron said that the UK had a plan in Afghanistan and it was up to the Taliban to choose how they would move forward.
"They have the choice - a political settlement where they give up weapons and become part of Afghanistan's political future or they take this attitude and Afghanistan will take care of its own security," he said.
He said that "big sacrifices" had been made since 9/11 with the UK losing more than 400 military personnel, but he added that progress had been made in making the world "safer".
He added: " I do think it is worth remembering that almost 3,000 people died here in New York, including 67 British citizens.
"The plots then were all coming from Afghanistan and Pakistan. And whereas a few years ago about 90% of the plots we were disrupting on the streets of London and New York and elsewhere were coming from the Pakistan/Afghanistan area, that is now maybe half that figure.
He described his relationship with Mr Obama as one of "good friends" and said the men were "working together with the aim of making our countries safer".
He later had a private lunch with the chief executives of several US companies - including Lloyd Blankfein of Goldman Sachs.