A US Embassy spokesman said Gen. James Mattis, head of US Central Command, arrived late Friday and was scheduled to meet army chief Gen. Ashfaq Pervez Kayani.

Ties between Islamabad and Washington are in crisis after American officials stepped up accusations that Pakistan’s premier spy agency was aiding insurgents in neighboring Afghanistan, including those who took part in an attack on the US Embassy last week in Kabul.

Kayani said on Friday that the charges were baseless, while other officials here have urged Washington to present evidence to back up such a serious allegation.

Pakistani Foreign Minister Hina Rabbani Khar warned the United States is risking losing an ally in the war on terror.

The row began when Adm. Mike Mullen, chairman of the Joint Chiefs of Staff, on Thursday accused the ISI agency of supporting Haqqani insurgents in planning and executing last week's 22-hour assault on the US Embassy and a truck bombing that wounded 77 American soldiers days earlier.

Kayani said the allegations were "very unfortunate and not based on facts."

The claims were the most serious yet by an American official against nuclear-armed Pakistan, which Washington has given billions in civilian and military aid over the last 10 years to try to secure its cooperation inside Afghanistan and against al-Qaida.

The Haqqani insurgent network is widely believed to be based in Pakistan's North Waziristan tribal area along the Afghan border. The group has historical ties to Pakistani intelligence, dating back to the war against the Soviets in Afghanistan in the 1980s.

The relationship between the two countries has never been smooth, but it took one of its hardest hits when US commandos slipped into Pakistan on May 2 without informing the Pakistanis of their mission and killed Al-Qaida chief Osama bin Laden in a garrison town not far from Islamabad.