Washington, Aug 30 - Pakistan illegally modified missiles given by the US for its defence to expand capability to strike land targets, a potential threat to India, a media report said Sunday.

The charge, which set off new tensions between the US and Pakistan, was made in an unpublicized diplomatic protest in late June to Prime Minister Yusuf Raza Gilani and other top Pakistani officials, The New York Times reported.

At issue is the detection by American intelligence agencies of a suspicious missile test April 23 - a test never announced by the Pakistanis - that appeared to give the country a new offensive weapon.

American military and intelligence officials say they suspect that Pakistan has modified the Harpoon anti-ship missiles that the US sold in the 1980s, a move that would be a violation of the Arms Control Export Act.

The accusation comes at a time, when the administration is asking the US Congress to approve $7.5 billion in aid to Pakistan over the next five years, and when Washington is pressing a reluctant Pakistani military to focus its attentions on fighting the Taliban, rather than expanding its nuclear and conventional forces aimed at India.

While American officials say that the weapon in dispute is a conventional one, the subtext of the argument is growing concern about the speed with which Pakistan is developing new generations of both conventional and nuclear weapons.

'There is a concerted effort to get these guys to slow down,' one senior administration official was quoted as saying. 'Their energies are misdirected.'

Pakistan has denied the charge, saying it developed the missile itself.

Whatever their origin, the missiles would be a significant new entry into Pakistan's arsenal against India. It would enable Pakistan's small navy to strike targets on land, complementing the sizable land-based missile arsenal that Pakistan has developed, the paper said.

That, in turn, would be likely to spur another round of an arms race with India that the US has been trying, unsuccessfully, to halt.

'The potential for proliferation and end-use violations are things we watch very closely,' said another senior administration official, who also spoke on condition of anonymity to New York Times.