The US hopes that Pakistan will agree to reopen the supply routes to NATO troops in Afghanistan as the Senate has threatened to link the aid to Pakistan with the country's cooperation on the issue.

Talks are ongoing and we hope to reach a resolution soon, an unnamed US official told Reuters.

Pakistan closed the supply lines last November after a NATO airstrike killed 24 Pakistani soldiers. Before closing down the routes, Pakistan had demanded an apology for the drone strikes and said that NATO drone strike in the country violated its sovereignty and international rules. However, the US declined to oblige Pakistan's demand for an apology and insisted that it should cooperate in the war against terrorism.

The blockade has forced NATO to depend on longer and expensive routes through Russia and Central Asia for its shipments of war supplies. The supply lines are crucial for NATO as it planned the withdrawal of the combat troops from Afghanistan by the end of 2014.

Pakistani President Asif Ali Zardari was invited in the last minute to a NATO meeting held in Chicago May 20 and 21 to discuss the issue. US President Barack Obama declined to have an official one-on-one meeting with Zardari in the NATO summit as a protest to Pakistan's action. However, he met Zardari on the sidelines of the summit briefly and told him that Pakistan needed to be part of the solution in Afghanistan, Reuters reported.

NATO leaders stressed upon Zardari to reopen the supply routes to the NATO troops while a US Senate panel threatened that it would cut the US aid to Pakistan if it did not reopen the supply routes.

Pakistan's unpopular government, which is facing strong public rebuttal for cooperating with the NATO forces, is finding it difficult to oblige the West. At the same time, the country, which heavily depends on US aid, finds it difficult to humiliate the US on the issue.

The Zardari government has said it is ready to reopen the supply lines but will charge $5000 for each truck that passes through its territory, a fee which the US and the NATO are saying is unreasonably high.

Meanwhile, Zardari is facing pressure in his country. Opposition parties and the hardline Islamic groups have warned the government against bowing down to Western powers.

The attitude of the heads of countries towards President Zardari during NATO summit - irrespective of 40,000 sacrifices rendered by Pakistani soldiers and civilians and $70 billion loss incurred to country's economy during the US waged so-called war on terror - was amounted to disgrace the whole nation, said Imran Khan, chairman Pakistan Tehreek-e-Insaf Party, The Nation reported.

Amid the ongoing crisis, a NATO drone strike Wednesday killed four militants in a Taliban stronghold in Pakistan, media reports said.

The drone fired two missiles on a house in the Tabai area near Miranshah. It is not immediately known if an important target is among those killed, one of the security officials told AFP on condition of anonymity.