WASHINGTON: The leaders of a US House-Senate negotiating panel said on Monday they had agreed to freeze $700 million in US aid to Pakistan until it provides some assurances of assistance in the fight against improvised explosive devices in the region.

The explosive devices are among militants' most effective weapons against US and coalition troops in Afghanistan.

Many are made using ammonium nitrate, a common fertilizer shipped across the border from Pakistan. The freeze on US aid was agreed as part of a defence bill that is expected to be passed this week.

The United States wants “assurances that Pakistan is countering improvised explosive devices in their country that are targeting our coalition forces,” Representative Howard McKeon, a House Republican, told reporters.

The United States has allocated some $20 billion in security and economic aid to Pakistan since 2001, much of it in the form of reimbursements for assistance in fighting militants.

But US lawmakers have expressed increasing frustration with Pakistan's efforts in the war.

There have been numerous proposals to make US aid to Pakistan conditional on more cooperation in fighting militants such as the Haqqani network Washington believes operate out of Pakistan and battle US troops in Afghanistan.

US lawmakers allege that many Afghan bombs are made with fertilizer smuggled by militants across the border from Pakistan into Afghanistan.

“The vast majority of the material used to make improvised explosive devices used against US forces in Afghanistan originates from two fertilizer factories inside Pakistan,” Senator John McCain, a Republican, said in the Senate last week.

The provision freezing $700 million in aid to Pakistan was agreed upon by leaders of the armed services committees from both parties in the House and Senate, including McCain. It is part of compromise legislation authorising US defence programs expected to be approved this week, McKeon said.

He said the bill would also require the Pentagon to deliver a strategy for improving the effectiveness of US aid to Pakistan.