The U.S. State Department says Secretary of State Hillary Clinton is to travel to Pakistan, but details on the timing of the trip are not being released due to security reasons.The U.S. special representative for Pakistan and Afghanistan, Richard Holbrooke, told reporters Friday in Washington that Clinton will meet with the leadership of Pakistan, as well as members of civil society, the opposition and the business community. He said there are constraints on how much of the trip can be discussed, including the timing of it. The announcement comes a day after the U.S. Senate passed a defense spending bill that includes military aid to Pakistan, but with restrictions that may upset Pakistani officials wary of U.S. interference.The bill imposes limits on how Pakistan gets reimbursed from a $1.6 billion fund used to support countries that help U.S. counter-terror efforts.One provision of it says that aid to Pakistan must not "affect the balance of power in the region" - meaning Pakistan should not use U.S. funding to build up defenses against India.It also says the U.S. Defense Department must certify that Pakistan is making "concerted efforts" to fight al-Qaida and the Taliban before it can receive the money.Pakistani officials complained that similar restrictions in a separate U.S. bill for non-military aid to Pakistan undermine the country's sovereignty. One contentious section of the $7.5 billion Kerry-Lugar bill requires the U.S. secretary of state to periodically verify that Pakistan's civilian government is exercising control over the military.Some information for this report was provided by AFP.