* Talks with Taliban collapsed before even started
* Karzai says wants talks to start as soon as possible
* PM says no UK combat troops in Afghanistan after end 2014 (Recasts, adds Karzai and Cameron news conference)
By Andrew Osborn
KABUL, June 29 (Reuters) - The chance to end 12 years of grinding war in Afghanistan should be seized by those committed to peace, British Prime Minister David Cameron said in Kabul on Saturday.
His comments come barely a week after talks between the Afghan state and the Taliban were announced, only to collapse within days due to perceptions that the Taliban were trying to reestablish themselves on the world stage.
Speaking at a news conference with President Hamid Karzai in the capital following a visit to British troops serving in the southern province of Helmand, Cameron said now was the time to pursue peace.
"There is a window of opportunity and I would urge all those who renounce violence, who respect the constitution, who want to have a voice in the future prosperity of this country to seize it."
Karzai also said peace talks with the insurgency were much desired, adding: "We hope the peace talks will begin as soon as possible."
"We want peace and stability in Afghanistan, we want the return of the Taliban back to their country, we want them to be part of this society and this policy and to work for their own country."
Taliban militants attacked buildings near the presidential palace this week, underscoring the difficulty of getting the talks off the ground.
TROOP WITHDRAWAL
Cameron's visit comes 11 days after a ceremony marking the start of the final phase of the handover of nationwide security responsibility to Afghan forces.
Dubbed "milestone 2013" by NATO, the event will lead to the departure of all NATO troops at the end of next year.
Britain is in the process of reducing the 7,000 troops it has in the country and of removing equipment before the end of the NATO-led International Security Assistance Force (ISAF) mission.
Cameron, on his 10th visit to Afghanistan as prime minister, confirmed Britain would cease to have any combat role in Afghanistan once the mission ends.
The United States and some other countries have said they will keep a small force of soldiers in Afghanistan for several years after 2014 to try to ensure stability and to pursue people they consider to be terrorists.
"The PM has been clear that we have paid a heavy price and already given a lot," a senior British government official said. Some 444 British troops have been killed since military action began in October 2001.
"Our combat troops will leave at the end of next year (and) the only military commitments we have made beyond 2014 are to part run the Officer Academy and to provide financial support to sustain Afghan forces," the official said.
The Afghan National Army Officers' Academy is a British project to provide Afghanistan with a military officers school similar to the Sandhurst Royal Military Academy in England.
British officials added that Cameron was keen to boost political stability ahead of a presidential election next year that Western diplomats hope will result in the first peaceful transition of power in Afghanistan since 1901. (Additional reporting by Mirwais Harooni; Writing by Dylan Welch; Editing by Alison Williams)
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