In a blunt message to Pakistan, United States Secretary of State Hillary Clinton has said that the US will never tolerate a safe haven for those who kill Americans and it wants Islamabad to take concrete action in the war against terrorism.

"The United States has clear expectations for this relationship. And as President (Barack) Obama said on Wednesday night, US will never tolerate a safe haven for those who kill Americans," Clinton said on Thursday in her testimony before the powerful Senate Foreign Relations Committee.

"We are looking to Pakistan to take concrete actions on the goals we share: defeating violent extremism, which has also taken so many innocent Pakistani lives; ending the conflict in Afghanistan and securing a stable, democratic, prosperous future," Clinton said.

"These are obviously tough questions to ask of the Pakistanis. And there are many causes for frustration. But we should not overlook the positive steps of recent weeks since May 2. Counterterrorism cooperation continues and several very key extremists have been killed or captured," she said.

Clinton said the US believes that no senior Pakistani officials knew of Osama bin Laden's presence. But she said it is possible that bin Laden was protected with a "wink and a nod" by lower-level officials.

"As I told the Pakistanis, America cannot and should not try to solve Pakistan's problems. They have to eventually do that themselves. But nor can we walk away from this relationship and ignore the consequences," she said referring to her recent visit to Pakistan.