Pakistan on Monday strongly condemned the increasing attacks by U.S. drone strikes on its territory. A drone strike in Pakistan's tribal areas killed eight suspected militants early Monday, Pakistani officials said, as the U.S. pushes ahead with the controversial drone program despite Pakistani demands to stop.

Three drone strikes in as many days on suspected militants have killed 27 people, Pakistani intelligence officials say.

The foreign ministry called the attacks "illegal" and said they violated the South Asian country's sovereignty.

Washington and Islamabad are deadlocked in negotiations over the re-opening of overland supply routes to NATO forces in Afghanistan.

Islamabad blocked the supply routes in November after 24 Pakistani soldiers were killed by cross-border "friendly fire" from NATO aircraft.

The supply lines are considered vital to the planned withdrawal of most foreign combat troops from Afghanistan before the end of 2014.

The Pakistan government says the CIA drone campaign fuels anti-American sentiment in the country, and is counterproductive because of the collateral damage - civilian deaths - it causes.

U.S. officials say such strikes by the remotely piloted aircraft are a highly effective way of attacking militants and are an important weapon in the war against militancy.

The ongoing attacks are also complicating efforts for the U.S. and Pakistan to come to an agreement over reopening the supply routes to NATO and American forces in Afghanistan. American airstrikes inadvertently killed 24 Pakistani soldiers in November, prompting Islamabad to block U.S. and NATO supply lines running through its territory.