Pakistan army sources have reportedly confirmed the presence of a large number of U.S. forces on the Afghanistan side of the border, preparing for a drone missile offensive against militants of the Haqqani network, a faction of the Taliban.

The U.S. military build-up includes helicopter gunships, heavy artillery and hundreds of U.S. and Afghan troops, according to a report in the British newspaper, The Daily Telegraph.

Tribal militiamen, fearing for their lives, apparently gathered in the city of Miranshah in North Waziristan to prepare their response to a potential coordinated attack.

Meanwhile, officials of the Federally-Administered Tribal Areas (FATA) warned that any border incursions by foreign troops onto Pakistani territory would be repelled by the army.

Officials in Washington have been pressuring Islamabad to upgrade their crackdown on Haqqani militants known to be operating in the lawless, mountainous border regions of North Waziristan. Haqqani have been blame for launching attacks against NATO targets in Afghanistan.

In response, Pakistani officials have countered that their military capacity has been maximized ion trying to quell the Pakistan Taliban which seeks to violently overthrow the Islamabad government.

Tensions between the US and Pakistan reached a new paradigm last month when Admiral Mike Mullen, the chairman of the U.S Joint Chiefs of Staff, openly accused Pakistan's ISI intelligence network of having direct ties with Haqqani and conspiring with the militant group to attack the US embassy in Kabul.

Pakistan military sources reportedly said that U.S. officials informed them of the build-up, suggesting some cooperation might have been reached with respect to mounting an offensive against Haqqani.

Haqqani is closely linked to the Taliban and has been described by the U.S. and one of the biggest threats to NATO forces in Afghanistan.