By Katherine Baldwin

KABUL, July 24 (Reuters) - Britain's new foreign secretary, David Miliband, landed in Afghanistan on Tuesday to encourage President Hamid Karzai's efforts to improve law enforcement and tackle corruption as British troops battle Taliban rebels.

On his first trip outside Europe since Prime Minister Gordon Brown promoted him to the post last month, Miliband will discuss how Karzai can build government capacity and extend his remit into Afghanistan's tribal areas, officials said.

Miliband's presence in Afghanistan so soon into Brown's new administration is designed to reassure Afghans their country is high on Britain's agenda and London's commitment is for the long haul, Miliband told reporters en route to Kabul.

He will also deliver Britain's core message to Karzai.

"Our agenda is no secret," he said. "It is to promote economic development, to squeeze the space in which terrorist groups can develop, tackle the narcotics problem, (and) promote good government and democratic institutions."

The two men will also discuss progress in fighting the Taliban and al Qaeda-linked insurgents who are especially active in the south and east, as well as counter-narcotics strategy.

Britain, which has 7,100 troops in Afghanistan and a large financial commitment in development aid, is only too aware of the consequences of failure of NATO's mission there and of efforts to extend democracy across the country.

Failure could turn Afghanistan into a breeding ground for al Qaeda militants and have a knock-on effect on Pakistan and potentially Iran, British officials say.

BIG CHALLENGES

In one of his final speeches last month, former British Prime Minister Tony Blair warned Afghanistan risked being overwhelmed by the same anti-Western violence that has torn up Iraq.

"Afghanistan embodies some of the biggest challenges for foreign policy," Miliband said.

"The challenges and problems are manifold," he said, but added it was important not to slip into fatalism about Afghanistan's prospects.

With the attempted bombings in Britain last month still fresh in mind, Miliband will also urge Karzai to work more closely with Pakistan's President Pervez Musharraf in order to stop the Taliban and other militants sheltering and training along the lawless border between the two countries.

Miliband's planned visit coincides with three days of mourning for former Afghan King Mohammad Zahir Shah who died on Monday, aged 92.

Miliband plans to attend Zahir Shah's funeral, to hold talks with Karzai and meet Afghan and British officials.

Last week, a British parliamentary committee highlighted a series of concerns about progress in Afghanistan, saying there were worrying signs the Taliban were growing stronger and that Afghan police and armed forces lacked training.

It also said the 36,000-strong ISAF mission needed reinforcements to battle the Taliban and al Qaeda militants that were expanding their influence in the south.

Britain leads NATO forces in the restive Helmand province in southern Afghanistan.

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