U.S.-led NATO forces in Afghanistan on Friday vowed to investigate an airstrike that President Hamid Karzai said killed a two-year-old boy, as acrimony deepens over a deal to allow U.S. troops to stay in the country after 2014.

Civilian casualties have been one of the most sensitive issues of the 12-year military intervention in Afghanistan, and Karzai warned that the latest incident threatened the proposed bilateral security agreement (BSA) with Washington.

U.S. national security advisor Susan Rice told Afghan President Hamid Karzai Monday that a delay in signing a troubled security deal risked the U.S. pulling troops out of the country completely next year.

The U.S. said that Karzai had called for "new conditions" for signing the bilateral security agreement (BSA) to allow U.S. forces to remain in the country after 2014.

U.S. national security adviser Susan Rice is in Afghanistan, where she will meet with President Hamid Karzai, the White House said Monday.

"She will meet the president at his invitation," National Security Council spokesman Patrick Ventrell told Agence France Presse, adding that the meeting was added to Rice's itinerary of "travel around country" and visiting with U.S. troops, development experts and diplomats.

Afghan President Hamid Karzai has set conditions for signing a security deal with the United States, including an end to military operations on Afghan homes and cooperation on peace and election processes, a spokesman said Saturday.

Karzai will explain the conditions to a grand assembly that will advise on whether to accept the deal in a closing speech planned for Sunday, presidential spokesman Aimal Faizy told AFP.

Afghanistan on Friday rebuffed a U.S. demand to sign a highly anticipated security pact as soon as possible, insisting the document must wait until after next year's presidential election.

Washington warned Kabul on Thursday to sign the Bilateral Security Agreement (BSA) pact as soon as possible, with senior officials hinting that delaying beyond the end of this year could mean no post-2014 U.S. troop presence.

Pakistan's Prime Minister Nawaz Sharif on Thursday met a high-ranking delegation from Kabul tasked with pushing forward Afghanistan's peace process, according to a statement from his office.

The three-member group representing the High Peace Council (HPC) arrived in Pakistan a day earlier on a mission that, according to Afghan President Hamid Karzai, was meant to include a meeting with Abdul Ghani Baradar, the Taliban's former deputy freed from jail in September.

U.S. Secretary of State John Kerry said Wednesday that he and Afghan President Hamid Karzai had reached agreement on the terms of a deal to cover the presence of American troops beyond 2014.

"As we sit here tonight we have agreed on the language that would be submitted to a loya jirga, but they have to pass it," Kerry said, talking about a draft text of a bilateral security agreement to be voted on by Afghan elders. He also stressed there was never any talk about the U.S. issuing an apology to Afghanistan for events over the past 12 years.

The United States has ruled out apologizing to Afghanistan for "mistakes" made during the 12-year war and denied claims in Kabul that such a mea culpa was being drafted.

The stern comments in Washington came after Afghan leader Hamid Karzai's spokesman said President Barack Obama planned to write a letter acknowledging that American military errors had caused civilian casualties.

Afghanistan and the United States have solved a key sticking point in a crucial security pact just two days before it was due to be voted on by Afghan tribal and political leaders, an Afghan official said Tuesday.

Aimal Faizi, President Hamid Karzai's spokesman, told reporters in Kabul that the deal would allow U.S. troops to enter Afghan homes once Nato forces withdraw in 2014 but only in "extraordinary circumstances" where there was an urgent risk to life. The compromise ends an impasse which had threatened to derail the agreement.

Afghanistan's finance minister on Monday rejected "doomsday" predictions that the impoverished country would plunge back into civil war when the bulk of US-led troops exit.

Many international observers have voiced fears Afghanistan could descend into civil war after the 2014 exit of Western combat forces -- similar to the conflict the country experienced in the 1990s.