Pakistan's army threatened on Thursday to reconsider its crucial cooperation with the United States if Washington carried out another unilateral attack like the killing of Osama bin Laden. In New York, President Barack Obama met firefighters and visited Ground Zero in Lower Manhattan to offer comfort to a city still scarred by the September 11, 2001, attacks masterminded by bin Laden that killed nearly 3,000 people. He said the killing of bin Laden by a U.S. commando team in Pakistan on Monday "sent a message around the world, but also sent a message here back home, that when we say we will never forget, we mean what we say. But a senior Pakistani security official said U.S. troops killed bin Laden in "cold blood," fueling a global controversy and straining a relationship that Washington deems key to defeating the al Qaeda movement that bin Laden led. Seeking to repair those ties, Secretary of State Hillary Clinton said in Rome on Thursday that Washington was still anxious to maintain its alliance with Islamabad. "It is not always an easy relationship," she said. "But, on the other hand, it is a productive one for both our countries and we are going to continue to cooperate between our governments, our militaries, our law-enforcement agencies, but most importantly between the American and Pakistani people." But Pakistan's army, facing rare criticism at home over the U.S. operation in Abbottabad, a town just an hour's drive from the capital, said in its first comment since the attack that Chief of Staff General Ashfaq Kayani had sent a stern warning. More Pakistanis say U.S. shot bin Laden in "cold blood"