U.S. State Department Spokesman Mark Toner said Monday that relations with Pakistan have some "very clear challenges" due to Islamabad's suspected ties with a militant group.

The remarks came at a time when the two countries' already fragile ties sunk to a new low after senior U.S. officials, including Chairman of U.S. Joint Chiefs of Staff Mike Mullen, explicitly accused Pakistan of backing the Haqqani network which was blamed for the recent attacks on the U.S. embassy in Afghanistan.

"I think it's clear that terrorism is a threat to both Pakistan and to the United States, and we're committed to working with the Pakistani Government to address it," Toner told reporters at a regular briefing, suggesting that Washington is considering designating the Haqqani network as a foreign terrorist organization.

"As far as our concerns about the Haqqani Network, that's been raised at the highest levels," he said, citing the recent remarks by U.S. State Secretary Hillary Clinton, Defense Secretary Leon Panetta and Mullen.

He said the status of the bilateral ties is "certainly a matter of concern," but he emphasized that Washington is addressing these concerns by "working constructively with the Pakistani government."

Toner said the U.S. government has slapped sanctions on the "kingpins" of the Haqqani network, including its financiers, leadership and dangerous operatives, suggesting that Washington is considering the designation of the group as the foreign terrorist organization.

"The idea that we haven't gone after the Haqqani Network at all, I think, is a mischaracterization," he added.

The recent U.S. harsh criticism has been met with outright denial and outrageous reaction from Pakistan which has warned that Washington may lose an ally of the war on terror.

U.S.-Pakistani relations have already deteriorated this year after tensions rose over a secret U.S. commando mission to kill al Qaida leader Osama bin Laden in Pakistan without the permission from Islamabad.