An Afghan man wearing an Afghan army uniform shot at US soldiers in south-eastern Afghanistan, killing at least one serviceman on Sunday, local officials and the Nato-led coalition said.

The so-called "insider attack" in Paktika province is the fourth in less than a month and is likely to strain already tense ties between coalition troops and their allies, with most foreign troops scheduled to withdraw by the end of next year. A Reuters tally shows Sunday's incident was the tenth this year, and took the death toll of foreign personnel to 15.

"A man wearing an Afghan army uniform shot at Americans in Sharana city [the provincial capital] near the governor's office," said an Afghan official, adding that two soldiers had been hit by the gunfire.

The Nato-led coalition confirmed one soldier had been shot by a man in security forces uniform, but did not comment on his nationality or whether the Afghan was wearing a army uniform.

Insider attacks threaten to further undermine waning support for the war among Western nations sending troops to Afghanistan. A similar flurry of attacks last year prompted the Nato-led force to briefly suspend all joint activities and take steps to curb interaction between foreign and Afghan troops.

That has cut down the number of incidents, but some soldiers say the measures have further eroded the hard-won trust painstakingly nurtured between the allies over more than 12 years of war.

Late on Saturday, the US Secretary of State, John Kerry, and President Hamid Karzai of Afghanistan announced they had reached preliminary agreement on a bilateral security pact that now depends on the approval of Afghanistan's tribal leaders.

The pact, announced jointly by Kerry and Karzai late on Saturday after two days of talks in the capital, Kabul, would keep some US forces in Afghanistan after the withdrawal of most foreign troops by the end of next year.

The draft includes a US demand to retain legal jurisdiction over its troops who stay on in Afghanistan, which would give them immunity from Afghan law. Karzai opposes that and said the question could not be decided by his government. Instead, a Loya Jirga, or an assembly of elders, leaders and other influential people, will consider the demand and decide whether to accept it.

The United States is insisting it cannot agree to a deal unless it is granted the right to try in the United States its citizens who break the law in Afghanistan. "We need to say that if the issue of jurisdiction cannot be resolved, then unfortunately there cannot be a bilateral security agreement," Kerry told a news conference.

Karzai said the recent US snatching of a senior Pakistani Taliban commander on Afghan soil was an example of the kind of action that Afghanistan wanted to avoid. "Our discussion today in particular has been focused on making sure that through the bilateral security agreement such violations are not repeated," he said.