Pakistani officials say suspected U.S. drone strikes have killed at least 38 militants in Pakistan's northwest in the last two days, in one of the heaviest barrages on suspected militant compounds in recent months.

The strikes targeted three suspected compounds late Monday and early Tuesday in the North and South Waziristan tribal areas, which border Afghanistan. The militants are believed to shelter inside Pakistan while conducting attacks on NATO forces in Afghanistan.

The United States has been pushing Pakistan to conduct an offensive against Taliban and al-Qaida-linked militants in North Waziristan. Islamabad has resisted, saying it must consolidate military gains made in other areas first.

U.S. officials have never publicly acknowledged the use of drone strikes inside Pakistan, but privately they have confirmed their existence to various news outlets. Pakistan has condemned the attacks as a violation of its sovereignty, but the strikes are believed to be carried out with the help of Pakistani intelligence.

The airstrike campaign has intensified since U.S. special forces killed Osama bin Laden in the Pakistani city of Abbottabad on May 2. The U.S. raid on bin Laden's compound, conducted without previously notifying Pakistani authorities, further strained relations between the two countries.