Pakistani officials say suspected U.S. drone strikes have killed at least 42 militants in Pakistan's northwest in the past two days, in one of the heaviest barrages on suspected militant compounds in recent months.

The strikes Monday and Tuesday targeted three suspected compounds and a vehicle 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.

Pakistan recently launched an offensive in the nearby Kurram tribal region to clear the area of militants and reopen a road connecting the upper and lower parts of the district. The United Nations says the process has displaced some 84,000 people.

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 air strike 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.