WASHINGTON - The United States has quietly restarted security assistance to Pakistan, US officials said on Sunday, after freezing much of that aid during a period of strained relations beginning with the 2011 Navy SEAL raid that killed al-Qaida chief Osama bin Laden.

While the move to free up the aid has been underway for some months, it became public as President Barack Obama prepares for a White House meeting on Wednesday with Pakistani Prime Minister Nawaz Sharif.

Relations between the two countries remain tense on major issues, including Pakistani opposition to US drone strikes and Washington's complaints about the ties of the Pakistani intelligence service to militant groups active in Afghanistan.

But the bonds appear to be on the mend after a series of major setbacks in recent years, including the bin Laden raid, a NATO air strike that mistakenly killed Pakistani border guards and a January 2011 incident in which a CIA contractor killed two men on the streets of Lahore, Pakistan.

Sharif is scheduled to meet with members of the US Congress in Washington this week and aid will be one of the main issues, a congressional staff member said.

The deterioration had led to freezing of some funding and to the US Congress enacting additional restrictions on aid to Islamabad.

"As part of our annual funding process, throughout the course of this past summer the State Department notified Congress of how it planned to program funds from several different accounts for various programs in Pakistan," State Department spokeswoman Marie Harf said.

"While this is part of a long process of restarting security assistance cooperation after implementation was slowed during the bilateral challenges of 2011 and 2012, civilian assistance has continued uninterrupted throughout," Harf said in an email.

The Associated Press first reported on Saturday that the United States was releasing $1.6 billion in military and economic assistance to Pakistan that previously had been appropriated.

Congressional aides said on Sunday the assistance had been restarted over the past few months.

For fiscal year 2014, which began on October 1, Obama has requested $1.162 billion for Pakistan, including $857 million in civilian assistance and $305 million in security assistance, Harf said.

Much of US security aid to Pakistan is intended to bolster the ability of its military to counter militants in the country's semi-autonomous tribal areas.