'When we have concerns, we act aggressively.'

A senior Pakistani official, also speaking on the condition of anonymity, said the US accusation was 'incorrect'.

The official said that the missile tested was developed by Pakistan, just as it had modified North Korean designs to build a range of land-based missiles that could strike India.

He said that Pakistan had taken the unusual step of agreeing to allow the US officials to inspect the country's Harpoon inventory to prove that it had not violated the law, a step that the US administration officials praised.

Some experts are also sceptical of the American claims.

Robert Hewson, editor of Jane's Air-Launched Weapons, a yearbook and Web-based data service, said the Harpoon missile did not have the necessary range for a land-attack missile, which would lend credibility to Pakistani claims that they are developing their own new missile.

Moreover, he said, Pakistan already has more modern land-attack missiles that it developed itself or acquired from China.

'They're beyond the need to reverse-engineer old US kit,' Hewson said in a telephone interview. 'They're more sophisticated than that'.

The dispute highlights the level of mistrust that remains between the US and Pakistani military that American officials like to portray as an increasingly reliable partner in the effort to root out the forces of the Taliban and Al Qaeda on its territory.

A central element of the American effort has been to get the military refocused on the internal threat facing the country, rather than on threat the country believes it still faces from India.

Pakistani officials have insisted that they are making that shift. But the evidence continues to point to heavy investments in both nuclear and conventional weapons that experts say have no utility in the battle against insurgents.

Over the years, the US has provided a total of 165 Harpoon missiles to Pakistan, including 37 of the older-model weapons that were delivered from 1985 to 1988, said Charles Taylor, a spokesman for the Defence Security Cooperation Agency.